Conversion of lens slit lamp photographs into physical light-scattering units

To derive from lens slit lamp photographs by means of densitometry the physically defined quantity for light scattering (the Rayleigh ratio) and to expand the use of the Lens Opacity Classification System (LOCS III) to include clear lenses and also to calibrate the LOCS III Nuclear Opacity (NO) score in physical terms.

Series of slit lamp photographs were taken from 38 eyes from 29 subjects (age range 18 to 84 years old) including cataracts, for 0.1- and 0.2-mm slit width, using 200 ASA and 1600 ASA film speed (Kodak professional; Eastman Kodak, Rochester, NY) and different flash settings with a Topcon SL-6E (12 slit/speed/flash combinations; Paramus, NJ). Additionally 19 eyes were photographed with a Zeiss 40 SL/P (8 slit/speed/flash combinations; Carl Zeiss, Thornwood, NY). A calibrated suspension of latex spheres also was photographed at the same 20 conditions. Densitometry was performed on the nuclear area of all photographs including the LOCS III standards, using a photometrically corrected photocell. Slit width and flash intensity settings were photometrically calibrated. All eyes and the suspension were digitally "photographed" with the EAS-1000 (Nidek, Gamagori, Japan) Scheimpflug system.

For each eye and the suspension, the series of 20 or 12 densities, corresponding to a range of about 1 log unit in the amount of light used, proved to follow closely a course common to all eyes (the two film characteristics), apart from a shift in the amount of light (because of the differences in light back scattering).

From normal slit lamp photographs, the physical quantity for light (back) scattering can be derived using transformation graphs derived in this study. The LOCS III NO score also can be used for clear lenses and translated into physical units. In this way, slit lamp photography can be used better for more precise studies, provided some minimal calibration of the photograph slit lamp.