Ultraviolet protection in eye and face protection against biological hazards.

BACKGROUND: In the development of eye and face protection standards against biological hazards, it is important to consider that the eye and face protectors may be used in the outdoor environment, where ultraviolet (UV) exposure from the sun is much higher than indoors. Workers affected include paramedics, ambulance officers and clinical testing personnel. METHODS: Spectral transmittances (280-780 nm) were measured on eight face shields and one goggle of the types used in hospital and clinical environments, three overhead projector sheets and an occupational impact protective goggle. The UV and luminous transmittances were calculated according to the international and two national sunglass standards, and compared with the compliance requirements. RESULTS: All the face shields and goggle lenses were made of variants of the same material. All the samples, including the overhead projector sheets, complied with the requirements of the sunglass standards (for normal conditions, in the case of the American standard). CONCLUSION: Since all the lenses complied with the sunglass standards for UV protection, and there appears to be uniformity in the choice of materials in this product type, there is no need to make provision, as is the case for occupational eye and face protection standards, for protectors that do not provide UV protection. This makes labelling and advice to end users from eyecare and safety professionals much simpler.

Recommendations and requirements for the wavelengths in Rayleigh equation anomaloscopes.

DIN 6160:2019 is a technical standard that sets requirements for Rayleigh equation anomaloscopes. Table 1 of the standard contains the limits for centroid wavelengths and spectral half power bandwidths (SHBW). The centroid limits are more restrictive than dominant wavelength recommendations. The SHBW limits have no known evidence base and are inconsistent between colors. The spectral characteristics of three commercial anomaloscopes brands were measured using a telespectroradiometer. Only the oculus instruments complied with DIN 6160 Table 1, but all the anomaloscopes complied with published recommendations. All complied with the DIN 6160 bandwidth requirements. This highlights the need to provide an evidence base for such requirements.

A model for assessing the efficacy of colour vision aids.

Optical filter aids are marketed which claim to improve colour discrimination in red-green colour vision defectives. An earlier model has been revised and used to assess 9 currently available aids. Spectral reflectances (400-700 nm) for 80 colours equally spaced in hue angle at four equally spaced saturations were synthesised from chromatically adjacent Munsell colours. Aid induced chromaticity changes for Protanomals and Deuteranomals were calculated. Five aids enhanced red-green discrimination significantly for Protanomals and six for Deuteranomals and one aid reduced it significantly for both defectives. Five aids enhanced blue-yellow discrimination in Protanomals and Deuteranomals for whom it is not needed.

Optical performance of welding curtains and existing standards.

Welding curtains and screens are intended to protect workers, other than the welder, from the effects of optical radiation generated by the welding process. The national and international standards for welding screens and curtains have different requirements. The aim is to compare the protection requirements of examples of welding curtain material and to assess compliance with the international and national standards. Spectral transmittance values (ultraviolet, visible, and infrared) of 21 samples were obtained from the records of an ISO/IES 17025 accredited test laboratory and performance/compliance was assessed according to each of the standards. In the ultraviolet, 10 samples passed and seven failed all standards. In the visible/infrared region, four samples passed and 10 failed all standards. Four samples passed the U.S. and international standards but failed the Australian/New Zealand standard in the blue-light transmittance requirement. One sample failed both the U.S. and Australian/New Zealand standards but the result for the international standard was borderline, one sample passed ISO but failed the blue-light requirements, and one failed ISO but passed the blue-light requirements. The derivations of the various requirements are not well documented. The Australia/New Zealand standard is significantly more stringent in the ultraviolet and blue-light regions. A review of the optical radiation hazards and revision of the standards are indicated. It is possible that curtains, other than those tested, that comply with the international standard might transmit hazardous levels of blue light and, conversely, adequate ultraviolet and blue-light protection is available with curtains that do not comply with the international standard.

Lighting for color vision examination in the era of LEDs: the FM100Hue Test.

Lighting conditions nominated for color vision testing are many and varied. The recommendation of CIE color rendering index (CIE CRI) ≥90 and correlated color temperature of close to 6500 K is widely made for color vision testing generally. With the demise of incandescent and fluorescent lighting and their replacement by light-emitting diodes (LEDs), this is an opportune time to revisit the recommendation. In this paper, we consider the current sources, acceptable and unacceptable, and improvements to the recommendation as it applies to the Farnsworth-Munsell 100 Hue Test (FM100Hue Test). We conclude that there is no need to treat LEDs as a special case but propose a modified CRI measure.

The blue light dose from white light emitting diodes (LEDs) and other white light sources.

PURPOSE: This study was carried out to determine if light emitting diodes (LEDs) represented a special case in exposing the retina to blue light, when compared with other methods of backlighting television and computer screens and with fluorescent tube lighting. METHOD: The spectral distribution of light from computer and television screens (cathode ray tube, plasma screen, organic LEDs, and screens backlit by cold cathode tubes and/or LEDs), domestic type spotlights (LED and fluorescent tube) and objects in the outdoor sunlit environment were measured with a telespectroradiometer. The spectral data were analysed for the proportion of blue light and luminance. The results were also presented as time to reach the occupational safety limit, without regard to the normal 10 000 s time limit by which time there is no summation of exposure. RESULTS: The amount of blue light in a source is essentially independent of the technology of the light source, but closely related to the correlated colour temperature. CONCLUSIONS: Fluorescent lamps show essentially the same proportions of blue light and LEDs do not represent a special case, given the same correlated colour temperature. Blue-blocking lenses are no more needed with LED sources than with other screen illumination methods or with fluorescent lighting. There is no evidence base on which to recommend blue-blocking lenses for indoor applications.

Limitations and Precautions in the Use of the Farnsworth-Munsell Dichotomous D-15 Test.

SIGNIFICANCE: Clinicians who administer the Farnsworth-Munsell D-15 test need to pay attention to the quality and quantity of lighting and the time that they allow for completion of the test, and all repeat attempts need to be included in reports on compliance with color vision standards. PURPOSE: The validity of the Farnsworth-Munsell D-15 has been questioned because practice may allow significantly color vision-deficient subjects to pass. In this article, we review the influence of practice and other factors that may affect the performance. These relate to both the design and the administration of the test. METHODS: We review the literature and present some calculations on limitations in the colorimetric design of the test, quantity and quality of lighting, time taken, and repeat attempts. RESULTS: In addition to the review of the literature, color differences and luminance differences under selected sources are calculated, and the increases in luminance clues under some sources and for protanopes are illustrated. CONCLUSIONS: All these factors affect the outcome of the test and need specification and implementation if the test is to be applied consistently and equitably. We recommend the following: practitioners should never rely on a single color vision test regardless of the color vision standard; lighting should be Tcp '' 6500 K and Ra > 90; illuminance levels should be between 200 and 300 lux if detection of color vision deficiency is a priority or between 300 and 1000 lux if the need is to test at the level where illuminance has minimal influence on performance; illuminance should be reported; time limits should be set between 1 and 2 minutes; repeat testing (beyond the specified test and one retest) should be carried out only with authorization; and initial and repeated results should be reported. A set of test instructions to assist in the consistent application of the test is provided in the Appendix.

Development of color vision discrimination during childhood: differences between Blue-Yellow, Red-Green, and achromatic thresholds.

Nonvisual demands of tests affect vision test results in children. 150 children (79 females and 71 males, 5.3-12.7 years of age) were examined. Isoluminant Blue, Yellow, Red, Green, and Black and White thresholds were established with a four-alternative forced-choice and pseudo-10-bit system with adaptive staircase and gaming elements. Where Threshold=b0+b1*age-1, b1 for RG=6.26±1.90 (95% confidence limits), Achr=3.96±1.07 and BY=12.48±2.76 were significantly different. The noncolor demands of the test are the same for RG, BY, and Achr, so the later development of BY discrimination is not an artifact of the test.

Normative values for a tablet computer-based application to assess chromatic contrast sensitivity.

Tablet computer displays are amenable for the development of vision tests in a portable form. Assessing color vision using an easily accessible and portable test may help in the self-monitoring of vision-related changes in ocular/systemic conditions and assist in the early detection of disease processes. Tablet computer-based games were developed with different levels of gamification as a more portable option to assess chromatic contrast sensitivity. Game 1 was designed as a clinical version with no gaming elements. Game 2 was a gamified version of game 1 (added fun elements: feedback, scores, and sounds) and game 3 was a complete game with vision task nested within. The current study aimed to determine the normative values and evaluate repeatability of the tablet computer-based games in comparison with an established test, the Cambridge Colour Test (CCT) Trivector test. Normally sighted individuals [N = 100, median (range) age 19.0 years (18-56 years)] had their chromatic contrast sensitivity evaluated binocularly using the three games and the CCT. Games 1 and 2 and the CCT showed similar absolute thresholds and tolerance intervals, and game 3 had significantly lower values than games 1, 2, and the CCT, due to visual task differences. With the exception of game 3 for blue-yellow, the CCT and tablet computer-based games showed similar repeatability with comparable 95% limits of agreement. The custom-designed games are portable, rapid, and may find application in routine clinical practice, especially for testing younger populations.

Evaluation of tablet computers for visual function assessment.

Recent advances in technology and the increased use of tablet computers for mobile health applications such as vision testing necessitate an understanding of the behavior of the displays of such devices, to facilitate the reproduction of existing or the development of new vision assessment tests. The purpose of this study was to investigate the physical characteristics of one model of tablet computer (iPad mini Retina display) with regard to display consistency across a set of devices (15) and their potential application as clinical vision assessment tools. Once the tablet computer was switched on, it required about 13 min to reach luminance stability, while chromaticity remained constant. The luminance output of the device remained stable until a battery level of 5%. Luminance varied from center to peripheral locations of the display and with viewing angle, whereas the chromaticity did not vary. A minimal (1%) variation in luminance was observed due to temperature, and once again chromaticity remained constant. Also, these devices showed good temporal stability of luminance and chromaticity. All 15 tablet computers showed gamma functions approximating the standard gamma (2.20) and showed similar color gamut sizes, except for the blue primary, which displayed minimal variations. The physical characteristics across the 15 devices were similar and are known, thereby facilitating the use of this model of tablet computer as visual stimulus displays.

Consistency of color representation in smart phones.

One of the barriers to the construction of consistent computer-based color vision tests has been the variety of monitors and computers. Consistency of color on a variety of screens has necessitated calibration of each setup individually. Color vision examination with a carefully controlled display has, as a consequence, been a laboratory rather than a clinical activity. Inevitably, smart phones have become a vehicle for color vision tests. They have the advantage that the processor and screen are associated and there are fewer models of smart phones than permutations of computers and monitors. Colorimetric consistency of display within a model may be a given. It may extend across models from the same manufacturer but is unlikely to extend between manufacturers especially where technologies vary. In this study, we measured the same set of colors in a JPEG file displayed on 11 samples of each of four models of smart phone (iPhone 4s, iPhone5, Samsung Galaxy S3, and Samsung Galaxy S4) using a Photo Research PR-730. The iPhones are white LED backlit LCD and the Samsung are OLEDs. The color gamut varies between models and comparison with sRGB space shows 61%, 85%, 117%, and 110%, respectively. The iPhones differ markedly from the Samsungs and from one another. This indicates that model-specific color lookup tables will be needed. Within each model, the primaries were quite consistent (despite the age of phone varying within each sample). The worst case in each model was the blue primary; the 95th percentile limits in the v' coordinate were ±0.008 for the iPhone 4 and ±0.004 for the other three models. The u'v' variation in white points was ±0.004 for the iPhone4 and ±0.002 for the others, although the spread of white points between models was u'v'±0.007. The differences are essentially the same for primaries at low luminance. The variation of colors intermediate between the primaries (e.g., red-purple, orange) mirror the variation in the primaries. The variation in luminance (maximum brightness) was ±7%, 15%, 7%, and 15%, respectively. The iPhones have almost 2× the luminance. To accommodate differences between makes and models, dedicated color lookup tables will be necessary, but the variations within a model appear to be small enough that consistent color vision tests can be designed successfully.

Effect of Ultraviolet Exposure on Impact Resistance of Ophthalmic Lenses.

PURPOSE: To investigate the effect of ultraviolet radiation (UVR) on the impact resistance of organic ophthalmic lens materials. METHODS: Plano power CR39, Phoenix, Trilogy, and polycarbonate lenses with various scratch-resistant (SR) and/or antireflection (AR) coatings were obtained in batches of 40 units. All lenses had a nominal thickness of 2 mm. Half of each batch was conditioned following the European Standard EN 168 protocol for the test of resistance to UVR (exposed group). The remaining lenses comprised an unexposed group for that combination of lens substrate and coating treatment. Each group was subjected to ballistic impact with 6-mm steel balls following the ZEST protocol to determine its mean breakage velocity. The difference in mean breakage velocity between exposed and unexposed groups of each combination of lens substrate and coating was assessed for statistical significance. RESULTS: Exposed uncoated CR39 showed a reduction in fracture velocity of 10.3 m/s whereas CR39 with ultra hard coat had a reduction of 3.5 m/s and CR39 with AR and SR coating had a reduction of 4.1 m/s. Scratch-resistant coated Phoenix had a reduction of 4.8 m/s whereas AR-coated Phoenix had a reduction of 3.7 m/s. The corresponding reductions for Trilogy were 3.9 and 17.8 m/s. All differences were significant at the p level of less than 0.05. Although we were unable to break unexposed SR-coated polycarbonate lenses with our test apparatus, exposed SR-coated polycarbonate had a mean breakage velocity of 142 m/s. CONCLUSIONS: Our data suggest that extended UVR exposure causes a significant reduction in the impact resistance of the ophthalmic lens substrates commonly used for occupational eye protectors. Protective lenses that have been exposed to high levels of UVR for extended periods should be replaced regularly to maintain optimal impact protection, even if they do not show visible damage owing to wear and tear.

Color Vision and the Railways: Part 1. The Railway LED Lantern Test.

PURPOSE: Lantern tests and practical tests are often used in the assessment of prospective railway employees. The lantern tests rarely embody the actual colors used in signaling on the railways. Practical tests have a number of problems, most notably consistency of application and practicability. This work was carried out to provide the Railway LED Lantern Test (RLLT) as a validated method of assessing the color vision of railway workers. METHODS: The RLLT, a simulated practical test using the same LEDs (light-emitting diodes) as are used in modern railway signals, was developed. It was tested on 46 color vision-normal (CVN) and 37 color vision-deficient (CVD) subjects. A modified prototype was then tested on 106 CVN subjects. RESULTS: All 106 CVN subjects and most mildly affected CVD subjects passed the modified lantern at 3 m. At 6 m, 1 of the 106 normal color vision subjects failed by missing a single red light. All the CVD subjects failed. The RLLT carried out at 3 m allowed mildly affected CVD subjects to pass and demonstrate adequate color vision for the less demanding railway tasks. Carried out at 6 m, it essentially reinforced normal color vision as the standard. CONCLUSIONS: The RLLT is a simply administered test that has a direct link to the actual visual task of the rail worker. The RLLT lantern has been adopted as an approved test in the Australian National Standard for Health Assessment of Rail Safety Workers in place of a practical test. It has the potential to be a valid part of any railway color vision standard.

Color Vision and the Railways: Part 2. Comparison of the CN Lantern Used on the Canadian Railways and Railway LED Lantern Tests.

PURPOSE: There are two currently available lantern tests that have their design based solely on the practices of the rail industry. These are the CN Lantern (CNLAN) used on the Canadian Railways and the Railway LED Lantern Tests (RLLT). In the same way that the signaling practices differ in the two jurisdictions, the design of the lanterns also differ. The CIE (Commission Internationale de l'Éclairage)-recommended color vision standards for transport predate both lanterns. The study was undertaken to assess the appropriateness of these lanterns in CIE Color Vision Standard 1. CIE Standard 1 is called "Normal color vision" but some very mild anomalous trichromats may pass the specified "lantern test that presents a high level of difficulty." METHODS: The lantern tests were undertaken by 46 color vision-normal and 37 color vision-deficient subjects. RESULTS: Subjects made more errors on the RLLT, and the pattern of errors is different, partly because there are blank presentations in the RLLT and "no light" is an acceptable response. The two lanterns showed agreement on the pass/fail categorization of 73 of the 83 subjects. The RLLT fails more color vision-normal subjects. CONCLUSIONS: Despite the different construction principles, the RLLT and CNLAN have pass/fail levels that are comparable with the Holmes-Wright Type B lantern, which is nominated in CIE Color Vision Standard 1 but is no longer commercially available. The higher failure rate of color vision-normal subjects on the RLLT has been addressed by changing the intensities of the two darkest red lights.

Color Vision and the Railways: Part 3. Comparison of FaLant, OPTEC 900, and Railway LED Lantern Tests.

PURPOSE: The Farnsworth Lantern (FaLant) and the OPTEC 900 are nominated in the Commission Internationale de l'Éclairage (CIE) Color Vision Standard 2. Neither test uses the railway signal color code of red, yellow, and green, and only the OPTEC 900 is commercially available. The Railway LED Lantern Test (RLLT) is based on railway signaling practices in New South Wales, Australia, and is nominated in the Australian railway medical standard. The objective of this study is to compare the performance of the three lantern tests. METHODS: The RLLT, FaLant, and OPTEC 900 were administered to 46 color vision-normal and 37 color vision-deficient (CVD) subjects. RESULTS: The pattern of errors on the RLLT was different from that of the FaLant and OPTEC 900. This may be accounted for, at least in part, by the different colors and the use of blank presentations in the RLLT. The three lanterns showed agreement in failing 21 and passing 6 of the CVD subjects (72.9%). The lanterns gave different results for 10 CVD subjects (27.9%): n = 5 passed only the RLLT and n = 3 passed only the FaLant; n = 1 failed only the FaLant and n = 1 failed only the RLLT. The overall failure rate by CVD for each lantern was 67.6% (RLLT), 73.0% (FaLant), and 78.4% (OPTEC 900). CONCLUSIONS: Despite the different construction principles, the pass/fail levels of the RLLT, FaLant, and OPTEC 900 are comparable and consistent with the performance of other lanterns listed by the CIE for Color Vision Standard 2. The RLLT may be a little easier to pass and is based on the signal color code used and actual signaling practice. We propose that the RLLT is also an appropriate lantern for CIE Color Vision Standard 2.

Recognition of simulated cyanosis by color-vision-normal and color-vision-deficient subjects.

There are anecdotal reports that the recognition of cyanosis is difficult for some color-deficient observers. The chromaticity changes of blood with oxygenation in vitro lie close to the dichromatic confusion lines. The chromaticity changes of lips and nail beds measured in vivo are also generally aligned in the same way. Experiments involving visual assessment of cyanosis in vivo are fraught with technical and ethical difficulties A single lower face image of a healthy individual was digitally altered to produce levels of simulated cyanosis. The color change is essentially one of saturation. Some images with other color changes were also included to ensure that there was no propensity to identify those as cyanosed. The images were assessed for reality by a panel of four instructors from the NSW Ambulance Service training section. The images were displayed singly and the observer was required to identify if the person was cyanosed or not. Color normal subjects comprised 32 experienced ambulance officers and 27 new recruits. Twenty-seven color deficient subjects (non-NSW Ambulance Service) were examined. The recruits were less accurate and slower at identifying the cyanosed images and the color vision deficient were less accurate and slower still. The identification of cyanosis is a skill that improves with training and is adversely affected in color deficient observers.

Visual function and retinal thickness in children with type 1 diabetes mellitus.

CLINICAL RELEVANCE: The possibility that changes in blue-yellow visual thresholds and some retinal thickness measures in children with diabetes mellitus may be observed before any visible fundus changes points to the possibility of these measures being a useful predictor that the risks of diabetic retinopathy are higher in some children than in others. INTRODUCTION: Previous studies showed mixed results on chromatic and achromatic contrast sensitivity early in the course of diabetes mellitus, and the findings of these studies may have been influenced by a lack of experimental sensitivity to visual deficits, a bias towards tritan-like errors or the cognitive demands of the tests and variations in sample composition. The purpose of this study was to evaluate colour and contrast thresholds and retinal thickness in children with type 1 diabetes mellitus compared with age-matched controls. METHODS: A prospective case-control study was carried out on 9-14-year-old children with type 1 diabetes mellitus (49 cases) and age matched controls (49) in which isoluminant red-green and blue-yellow and achromatic luminance contrast thresholds were measured. Fundus photography was used to grade diabetic retinopathy. Retinal thickness parameters were measured using optical coherence tomography. Data on the duration of diabetes mellitus, glycaemic control (HbA1c), blood glucose level, body mass index, blood pressure and blood oxygenation at the time of testing were obtained. RESULTS: The cases mostly had poorly controlled diabetes, HbA1c 8.6% (6.4-12.8%), for an average (range) duration of 5 (0.4-12) years. The cases had significantly higher blue-yellow thresholds (p = 0.02) and greater total retinal and inner retinal thickness (p < 0.05) than controls. No cases had diabetic retinopathy. Within the cases, poorer visual function and systemic health measures were associated with thinner retinal structures and greater global loss volume percentage in the ganglion cell complex. CONCLUSION: Blue-yellow thresholds of cases were raised compared to normal. Within the cases, higher luminance contrast thresholds were also associated with, mostly, ganglion cell complex reductions.

Difficult at dusk? Illuminating the debate on cricket ball visibility.

OBJECTIVES: Investigate the visibility of new and old red, white and pink cricket balls under lighting and background conditions experienced during a day-night cricket match. DESIGN: We modelled the luminance contrast signals available for a typical observer for a ball against backgrounds in a professional cricket ground, at different times of day. METHODS: Spectral reflectance (light reflected as a function of wavelength) was derived from laboratory measurements of new and old red, white and pink balls. We also gathered spectral measurements from backgrounds (pitch, grass, sightscreens, crowd, sky) and spectral illuminance during a day-night match (natural afternoon light, through dusk to night under floodlights) from Lord's Cricket Ground (London, UK). The luminance contrast of the ball relative to the background was calculated for each combination of ball, time of day, and background surface. RESULTS: Old red and old pink balls may offer little or no contrast against the grass, pitch and crowd. New pink balls can also be of low contrast against the crowd at dusk, as can pink and white balls (of any age) against the sky at dusk. CONCLUSIONS: Reports of difficulties with visibility of the pink ball are supported by our data. However, our modelling also shows that difficulties with visibility may also be expected under certain circumstances for red and white balls. The variable conditions in a cricket ground and the changing colour of an ageing ball make maintaining good visibility of the ball a challenge when playing day-night matches.

How practitioners say they answer the questions of patients about ultraviolet protection.

Patients should be able to rely on optometrists and optical dispensers to provide evidence-based answers to their questions on eye protection, including against ultraviolet radiation. Surveys indicate that there is public concern about the need for protection against ultraviolet radiation, particularly in sunlight. This investigation aimed to evaluate the quality of information provided by practitioners in response to typical questions to which they might reasonably be expected to have ready answers. A cross-sectional study was conducted of on-site responses from attendees of a 3 day optical fair, in Sydney, Australia, in 2017. Subjects were not forewarned about the study. The portion of the questionnaire reported here comprised open answers to four questions intended to represent typical enquiries of patients. The questions were about recommendations for ultraviolet protection in the context of 1) computer use, 2) outdoor use, 3) driving and 4) under office lighting. Eighty-three participants returned completed questionnaires out of 140 issued (61% response). The open-answer question responses were grouped into appropriate, borderline (mostly appropriate but mixed with non-UVR related recommendations) and others (mostly non-UVR related recommendations) and by job title. The proportion of appropriate answers to the four questions were 20%, 30%, 12% and 15%, respectively, which did not differ by job title. A significantly higher proportion of optical dispensers than optometrists selected 'Don't know/no answer' for all questions except office use. Eyecare professionals need to be educated on eye protection against UV radiation to improve the quality of information to be given to the public.

Luminance contrast preferences of people with a vision impairment for elements in the built environment.

BACKGROUND: Standards for building elements recommend a minimum luminance contrast of 30%. The basis of this value and the metric originally used is not known. OBJECTIVE: To begin to provide an evidence base for the specification of minimum contrast in building elements. METHODS: Subjects with and without a vision impairment were characterized by visual acuity, contrast sensitivity, visual fields and color vision. On an iPad they rated the visibility, as a function of contrast, of simulated door frames, door handles, light switches and stair nosings as "not visible at all", "poorly visible", "easily visible" and "extremely easily visible". RESULTS: The contrasts for each level of visibility were highly correlated with visual acuity and contrast sensitivity. A principal component analysis also verified the importance of visual acuity, contrast sensitivity and visual fields in rating visibility of simulations of building elements. The required contrast for door handles, light switches and stair nosings to attain the same ratings of visibility were very similar but less contrast was required for door frames. CONCLUSIONS: 30% Michelson contrast for building elements renders building elements only poorly visible for those with severe vision impairments. 65% luminance contrast is necessary for all elements to be "easily visible". Some increase (not a decrease) on the present 30% requirement and encouragement to exceed this requirement would seem appropriate. The use of simulated objects facilitates a systematic examination of the effect of contrast, but the applicability of the results to real-life remains to be demonstrated.