VISION RESEARCH ABSTRACTS Some abstracts that might be of interest

Phaco-Emulsification Causes the Formation of Cavitation Bubbles

Bengt Svensson (Umea, Sweden) & John Mellerio
Current Eye Research, 13:649-653, 1994
There have been reports of complications arising from damage to non-lenticular ocular tissue during the increasingly popular procedure of cataract extraction with phaco-emulsification. One cause of this damage might be the formation of cavitation bubbles. Such bubbles are known to produce free radicals and shock waves. This paper demonstrates directly the formation of cavitation bubbles at the tip of the phaco-probe. It also shows the importance of a smooth probe profile in reducing bubble formation. Recommendations are made for probe and tip design and for the use of minimum power during the surgical procedure of phaco-emulsification.

The figure shows the end of the phaco-probe: A, at the end of the advance stroke; C, at the end of the retraction stroke - note cavitation bubbles. B is half way through the advance stroke. Frequency was 44kHz .

Traffic Signal Light Detection through
Sunglare Filters of Different Q Factors

David A Palmer, John Mellerio & Amanda Cutler
Color Research & Application, 22:24-31, 1997
Recent guides for use of sunglare protection filters have introduced the concept of Q factors as a measure of colour appearance distortion in viewing traffic signal lights. The adoption of Q factor values was apparently arbitrary and not firmly based on experimental data. The manner in which changes in Q factor affect colour perception and detection of signal lights has been measured and shows that detection thresholds vary with the Q factor in a manner which can only partly be explained but which is not independent of the colour of the signal as assumed in the guides.

LED's VIEWED THROUGH COLOURED LENSES

John Mellerio & David Palmer
Proc CIE Symp on Standard Methods for Specifying & Measuring LED Characteristics , CIE publication x013, ISBN 3900734852, Vienna, 1997

The chromaticities and contrasts of a yellow LED simulating a traffic signal and of an instrument panel indicator (simulated by mixing light from two LED's) were calculated for viewing through three popular sunglare protection or driving lenses. Colour naming and quality rating of the lights confirmed the theoretical changes in chromaticity and contrast.

For example, one driving lens reduced the contrast of the yellow LED traffic signal to a third of that experienced with naked eye viewing, and halved the detection distance in a typical viewing situation.

Designers of systems that employ LED's should ensure that the visual information which they hope to convey is not degraded to a dangerous extent for those
who wear sunglasses or driving lenses.

Macular Pigment Measurement with a Novel Portable Instrument
MELLERIO, J., PALMER, D.A. & RAYNER, M.J. 
European Association for Vision and Eye Research, Palma de Mallorca, October 1998

PURPOSE   
 ‘    Hammond, Wooten & Snodderly (1997) suggested that Age Related Macular Degeneration (ARMD) might be correlated with the Macular Pigment Optical Density (MPOD)
‘    Thus it may be clinically useful to know a person’s MPOD in order to prognosticate maintenance of visual function
‘    Macular Pigment (MP), which is yellow, is a mixture of carotenoids only obtainable from the diet
‘    Knowledge of a patient’s MPOD could be used to advise about diet in order to improve protection against long-term oxidative light damage
‘    A practical screening method for determining MPOD in a clinician’s office is therefore desirable 
PRINCIPLES   
‘    Heterochromatic Flicker Photometry (HCFP) is a well established technique for measuring MPOD (Werner & Wooten, 1979)
‘    A flickering green light, which is not absorbed by MP, is matched in a test field with an adjustable flickering blue, which is absorbed by MP (the green and blue lights flicker in counter phase)
‘    The matches are made either with the retinal image of the test field on the fovea  (Case 1) or on the parafovea (Case 2) outside the pigmented area
‘    At the match point of minimum perceived flicker the brightness of the blue and the green will be the same
‘    The blue light is absorbed by the yellow MP so in Case 1 the subject needs more blue than in Case 2
‘    Provided that the green is constant it follows that:   
            MPOD = log B1 - log B2
    where B1 and B2 are the mean blue luminance values
‘    The flicker frequency must be high enough not to stimulate the rods - above about 12 Hz
‘    The blue-green match is made with the medium and long wave cones which are assumed to be evenly distributed across the central areas of the retina
‘    Any contribution of the short wave cones and the rods to the matches is prevented by adapting them with a blue background 

DEVICE   
‘    A novel method of obtaining the stimuli and background is to use Light Emitting Diodes (LED’s) as sources
‘    The drawing and photographs show a small portable prototype device using LED’s
‘    The test field, 0.5 deg diameter, is formed by transilluminating a circular aperture behind which is a small integrating box containing the blue and green LED’s

‘    The 5 deg adapting background is formed by an array of seven blue LED’s filtered to narrow the bandwidth: the light is passed through a partial diffuser and projected through a field lens before reflection from a glass plate in front of the test field 
‘    The luminance of the test field is about 50 cd.m-2 and that of the background is about 10 cd.m-2
‘    The flickering diodes are supplied by a constant current source developed by Millar and Barnet (QMW College, London): this source has been carefully designed so that the LED luminances are approximately linear with respect to the control scale values: Figure 1 shows the calibration curve for a green LED
‘    Figure 2 shows the normalised spectral power distributions of the three LED types used in the prototype together with the absorption of macular pigment
RESULTS   
‘    The measures of MPOD show distinct differences between individuals which are consistent from day to day: a few subjects have also been measured by an objective method which yielded values similar to those from the LED prototype
‘    The mean MPOD for 12 subjects (ages 19 to 65) was 0.287 ±0.136 sd which is similar to reports in the literature
‘    Figure 3 indicates a gender difference (p = 0.023) with male MPOD lower than that of females, contrary to previous reports (Hammond et al, 1996)
‘    Some subjects were good at making HCFP matches but even poor subjects could obtain useful results with a minimum of ten pairs of fovea/parafovea observations 
CONCLUSION                                           
‘    By using a novel combination of cheap, commercially available LED’s, a small integrating box and semi-Maxwellian background projection, we have made a practical system for measuring MPOD which is small and portable
‘    With appropriate development this would be suited to automated clinical screening in a wide range of environments

    References
    Hammond, BR, et al. Individual variations in spatial profile of human macular pigment.   J. Opt. Soc. Am. A., 14:1187-1196, 1997
    Hammond, BR, et al. Sex differences in macular pigment optical density, etc. Vision Res, 36:2001-2012, 1996
    Werner, JS & Wooten, BR. Opponent chromatic mechanisms etc.  J. Opt. Soc. Am., 69:422-434, 1979


THE DESIGN OF EFFECTIVE OCULAR PROTECTION FOR CW RADIATION SOURCES
in: Measurement of Optical Radiation Hazards: A Reference Book   International Commission on Non-Ionising Radiation Protection,
CIE Publication x016-1998, ISBN 39804789-5-5, 1998

Of the three strategies for ocular protection - avoidance, reduction of exposure time and attenuation of radiation at the eye - the last is the most popular for relatively less intense CW sources. It is also the most difficult to design because not only must it protect safely against chronic accumulative effects that may take many years to become apparent, it must allow at least some visual function to be maintained. The physical realisation of this protection is usually some form of lens fitted to a goggle, visor or spectacle. The designer must consider the spectral power distribution (spd) and likely duration of the radiation, and
its irradiance in absolute units. He must also take into account the action spectrum of each ocular structure and compare this with the spd to see if the effects of the radiation are likely to be harmful. If they are, he must then specify a filter with a spectral transmittance that reduces the transmitted radiation to safe levels.

It is this process that is a challenge for standards committees seeking to develop standards criteria for ultraviolet, visible or infrared protection against solar radiation for use in sunglasses and prescription eyewear. In this area there is continuing debate on whether any protection is essential or whether it should be left to the manufacturer to voluntarily build- in protection. However, with the advent of health criteria and occupational exposure limits, it has been possible more readily to specify spectral attenuation factors for welding, cutting, arc-lamps and foundry operations. There have been debates on the level of specification to spectral detail in such efforts but the problems seem more tractable.

Whatever the radiation source for which protection is sought, there follow two further stages in the specification of a protecting filter. The first is to ensure that the spectral transmittance of the filter is physically realisable and stable, and secondly to ensure that the attenuation of the visually effective radiation (light) is not such as to reduce visual function to below acceptable limits.

In this paper these stages will be considered in the light of attempts to produce standards, and attention will be given to the way that safe filters may prejudice visual performance by reducing contrast thresholds, visual acuity and color recognition. There is always the danger that the reduction of one hazard is replaced by another especially if standards are oosely drawn up.

LIGHT-EMITTING DIODES (LEDs) AND LASER DIODES:
IMPLICATIONS FOR HAZARD ASSESSMENT

International Commission On Non-Ionizing Radiation Protection (Health Physics, 77:218-220, 1999)

This statement is based upon the deliberations of the ICNIRP Standing Committee IV ("Optics") and was extensively discussed in a task group meeting of experts convened by ICNIRP which took place on 23-25 September 1998 at the University Eye Clinic, Regensburg, Germany. The following experts participated in this meeting:
W. Cornelius (Australia), D. Courant (France), P. J. Delfyett (USA), S. Diemer (Germany), W. Horak (Germany), G. Lidgard (UK), R. Matthes (Germany), J. Mellerio (UK), T. Okuno (Japan), M. B. Ritter (USA), K. Schulmeister (Austria), D. H. Sliney (USA), B. E. Stuck (USA), E. Sutter (Germany), and J. Tajnai (USA).

CONCLUSIONS AND RECOMMENDATIONS
It is concluded that all surface emitting LEDs and IREDs will be judged safe by applying the ICNIRP ELs for incoherent radiation as well as by the recommendations of CIE TC 6-38 (Lamp Safety) for realistic viewing conditions. This conclusion applies to any LED device which does not have optical gain. Only because of the extraordinary worst-case assumptions built into some current product safety standards, could one reach the conclusion that an LED or IRED poses a retinal hazard. On the other hand, the use of laser ELs to evaluate LEDs could result in an understatement of the lenticular risk if the source is very large and the lens becomes overheated.

It is therefore recommended that safety evaluations and related measurement procedures for LEDs follow the guidelines for incoherent sources (ICNIRP 1997). This approach provides the most accurate assessment of incoherent sources without problems originating from certain underlying assumptions incorporated into the limits developed for collimated laser beams. Diode lasers and VCSELs clearly should be treated in all standards as lasers. 

It is recognized that the determination of appropriate viewing durations and distances under different conditions of use is needed for any optical radiation hazard assessment. Unfortunately, not all safety guidelines currently recommend use of the same measurement distances and viewing durations. The future development of application-specific safety standards which may be applied to realistic viewing conditions will also contribute to reducing unnecessary concerns regarding LED and IRED safety.

    International Commission on Non-Ionizing Radiation Protection (ICNIRP).
    Guidelines on limits of exposure for broad-band incoherent optical radiation (0.38 to 3 :m)
    Health Phys. 73:539-597; 1997. 

Characteristics of Macular Pigment May be Used to Measure Pigment Density Psychophysically

EXECUTIVE SUMMARY OF THE SPECIAL INTEREST SYMPOSIUM:
Xanthophyll Carotenoids and the Macular Pigment
European Association for Vision and Eye Research Conference
10 – 13 October 2001, Alicante, Spain

Professor Mellerio explained, “The mixture of lutein and zeaxanthin that is macular pigment has well determined optical characteristics.” Macular pigment density can easily and non-invasively be measured in the human eye. Most of the different measuring techniques available rely on the principle that blue light is absorbed by the yellow macular pigment i.e. the more yellow pigment present, the more blue light is absorbed.

Various psychophysical techniques are utilised to measure the quantity of pigment in the living eye. These include colour matching, motion anomaloscope, spectral sensitivity and heterochromatic flicker photometry (HCFP) which is the most common principle employed in these psychophysical methods. Professor Mellerio presented a portable instrument that can be used to measure macular pigment densities in field studies or in the optometrist’s or ophthalmologist’s practice.

MACULAR PIGMENT OPTICAL DENSITY MEASUREMENTS WITH A PORTABLE SCREENING INSTRUMENT
                             
MELLERIO  J1,  van KUIJK E2,  PAULEIKOFF D2,  AHMADI-LARI S3, MARSHALL J3
1.  University of Westminster, London (UK)
2.  Institute of Ophthalmology, London (UK)
3.  Dept. Ophthalmology, St Thomas' Hospital, London (UK)

(See  MELLERIO, J., AHMADI-LARI, S., VAN KUIJK, F.J.G.M., PAULEIKHOFF, D., BIRD, A.C. & MARSHALL, J. A portable instrument for measuring macular pigment with central fixation. Current Eye Res, 25:37-47, 2002  for full publication of this work)

European Association for Vision and Eye Research Conference
10 – 13 October 2001, Alicante, Spain

Purpose  To see if macular pigment (MPOD) in the eyes of normal subjects and AMD patients varies with age, gender, eye colour, smoking habit, diet, chronic exposure to sunlight and disease state of the retina.
Methods  The portable screening instrument (described at EVER 1999) uses light emitting diodes and a free viewing condition, and employs heterochromatic flicker photometry.  107 subjects with normal vision measured MPOD in each eye and answered a simple questionnaire designed to find their age, gender, eye colour, smoking habit, what diet they ate and their exposure to sunlight.  These last two parameters were only crudely reported as more or less than 16 servings of vegetables, fruit and eggs per week and less than 3 hours on average per day exposure to sunlight or more than 3 hours plus a sunbathing habit.  MPOD was also measured in 17 patients with AMD and in age and gender matched controls.
Results  The mean MPOD for healthy subjects was 0.41±0.16 sd.  There was no significant correlation of MPOD with age but there were significant differences (p<0.05) for gender, iris colour and smoking habit similar to those reported in the literature.  The group of subjects with the diet poorer in vegetables, fruit and eggs had a significantly smaller MPOD than those with a richer diet.  Subjects with a greater exposure to sunlight had significantly less MPOD than those with the smaller exposure.  Male patients with advanced AMD had significantly less MPOD compared to those with healthy eyes and although the female eyes showed a similar trend, the difference did not reach significance.
Conclusions  The findings support those reported in the literature for age, gender, eye colour, smoking habit and richness of diet in carotenoids.  The reduction of MPOD with sunlight exposure and with advanced AMD in males is interesting and requires further investigation.

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