Hadiya Farhath.P, B S Optometry

Research Optometrist, Medical Research Foundation, Sankara Nethralaya, Chennai

The C-Quant device is a straylight measurement meter that relies on a psychophysical procedure using flickering stimuli¹. Straylight is defined as a point light source scattering in the eye’s optical media that creates a veil of light over the retina^{2, 3}.

What is the psychophysical approach in C quant? The answer is that the stimulus should consist of:

1. A circular test area divided into  two halves (diameter of 3.3 degree)
2. An area of constant luminance surrounding the circular test area
3. The white light of average luminance 25cd/m² (both halves and surrounding area)¹

Is this stuff simple? Yes, now the subject must be ready to identify the flickering light and the reference light (two-alternative forced-choice method) in which the flickering light of initial frequency 8Hz is randomly projected in either of the halves for a maximum of 6 seconds. This overall test would take approximately 5minutes for both the eyes¹.This is all about the psychophysical procedure of C-Quant.

Why is it so important to measure the straylight? Straylight can reduce the quality of vision or optical performance^{2, 4}. Without knowing the actual cause that is affecting the quality of vision, we will uncertainly end up giving the wrong treatment.

Is the straylight/light scattering in the eye asymptomatic? No, it does create symptoms such as disability glare, face recognition problems, hazy vision, reduced colour, and contrast sensitivity². Assessing straylight is particularly important for drivers, cataract patients, pre/post-refractive surgery, corneal cases, retinal cases, and elderly people driving especially in the night times^{5, 6}.

C-quant had solved the problem of direct assessment of intraocular straylight instead of measuring the effect of perception (visual acuity, contrast sensitivity, glare assessment)^2,5. For example, in some people whose visual acuity was 0.5Log MAR unit, C-quant value was increased more than four times compared to normal⁶. Here is the most frequent question of the patients we came across in our clinics: “When can I undergo surgery?” The answer is that the eye care professional cannot decide on surgery unless the seriousness of symptoms is known (especially in the patients with 6/6 vision). Depending on the straylight values, the seriousness of the subject’s symptoms can be considered by an eye care professional². It acts as an extra diagnostic tool over visual acuity⁶.

According to Van den Berg et al even, the clearest old natural lens is the important source of straylight and it will drop to levels of young in the pseudophakic eyes. He also adds that higher straylight values can be noted in pseudophakic eyes with early cataract changes, vitreous turbidity, laser surgery, and further studies need to be carried on materials of IOL, types of IOLs, corneal dystrophies, etc.²

In conclusion, it is clear that assessing straylight with C-Quant is very important to determine the quality of vision in both pre and post-surgical eyes. This C-quant value will help an eye care professional to counsel the patients as well as to decide on the surgery.

References:

1. Van den Berg, T.J., et al., Psychophysics, reliability, and norm values for temporal contrast sensitivity implemented on the two alternative forced choice C-Quant device. Journal of biomedical optics, 2011. 16(8): p. 085004.
2. Van Den Berg, T.J., et al., Straylight effects with aging and lens extraction. American journal of ophthalmology, 2007. 144(3): p. 358-363. e1.
3. Van Den Berg, T.J., Analysis of intraocular straylight, especially in relation to age. Optometry and vision science: official publication of the American Academy of Optometry, 1995. 72(2): p. 52-59.
4. Pompea, S.M., The management of stray radiation issues in space optical systems. Space Science Reviews, 1995. 74(1-2): p. 181-193.
5. Van Rijn, L., et al., Measurement of stray light and glare: comparison of Nyktotest, Mesotest, stray light meter, and computer implemented stray light meter. British Journal of Ophthalmology, 2005. 89(3): p. 345-351.
6. Van Den Berg, T., L. Franssen, and J. Coppens, Performance of the C–Quant Instrument for Retinal Straylight Assessment. Investigative Ophthalmology & Visual Science, 2006. 47(13): p. 1220-1220.