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function [fractionBleached] = ComputePhotopigmentBleaching(intensity,receptortype,units,source)
% [fractionBleached] = ComputePhtopigmentBleaching(intensity,units,source)
%
% Compute fraction of photopigment bleached, given some measure of
% the intensity of light reaching the eye.
%
% As far as I can tell, the fundemantal measurements of the half-bleach
% constant for cones were made by Rushton and Henry (1968, Vision Reserch, 8, 617-631).
% This fact I learned from CVRL (http://www.cvrl.org/database/text/intros/introbleaches.htm).
%
% I am pretty sure that the Rushton and Henry measurements were made for 560 nm
% light, and they give (see their Figure 2) a half-bleach constant of 4.3 log10 trolands (20,000 td).
% This number is also given in Boynton and Kaiser, Human Color Vision, 2nd edition,
% pp 211 and following.
%
% It's probably fine to compute bleaching for L and M cones given retinal illuminance
% in trolands, given that these are effects that matter over log10 units. But trolands
% are not going to help much for the S-cones. According to CVRL there aren't good
% measurements for the half-bleaching constant for S cones because putting enough short-wavelength
% light onto the retina to bleach the S cones is not good for the eyes.
%
% None-the-less, it seems nice to have this routine written so that it will return a number
% if you give it intensity either in trolands or in isomerizations/cone-sec. For 560 nm
% light and the CIE 10 deg fundamentals, I compute that 1 td is 137 isomerizations/cone-sec
% for L cones and 110 isomerizations/cone-sec for M cones. Take the weighted average value
% of (2*L + 1*M) = 128 and multiply by (10.^4.3) to get a half-bleach constant in
% isomerizations/cone-sec of 2.55e+06 (6.4 log10 isomerizations/cone-sec).
% [Computations done 6/2/14 using IsomerizationsInEyeDemo and setting the fundamentals to 'CIE10deg' and wavelength to 560 nm
% by hand in the code. These are for the 'Boynton' source.]
%
% [ASIDE: I used 10 deg fundamentals because I figure that Rushton's measurements are based
% on a fairly large field. Because the macular pigment absorbs a fair amount of light,
% this matters. If I compute instead with 2-deg fundamentals, I get that 1 td is 23.7
% L cone isomerizations/cone-sec and 19.5 M cone isomerizations/cone-sec. These two
% numbers are ballpark consistent with Rodiek page 475 who gives 18.3 and 15.9 for a
% monochromatic 540 THz light (555 nm)].
%
% This routine will do the computation either on the basis of input in trolands or input
% in isomerization/cone-sec, using the appropirate constant as above. Note that the
% computation of isomerizations takes into account lens and macular pigment, while the
% troland value is the straight troland value. A second advantage of using units of
% isomerizations/cone-sec is that you can compute this for other regions of the visual
% field and presumably the numbers will be about right. You can also compute for S-cones
% on the assumption that the half-bleach constant is the same for S-cones as for L- and M-
% cones.
%
% As far as I can tell, the computations and analysis of bleaching do not take into account
% changes in isomerization rate that occur because of change in spectral sensitivity of cones
% with bleaching. That is, the measurements are simply of steady state pigment density and are
% modeled with a formula that assumes monochromatic light (see treatment in Boynton).
%
% receptortype
% 'cones' -- computations for cones. [Default]
%
% units
% 'troalands' -- input intensity in trolands. Note that the computation only makes
% sense for L and M cones if this is the input. This is photopic trolands
% if receptor type is 'cones'. [Default]
% 'isomerizations' -- nominal isomerization rate in isomerizations/cone-sec, comptued
% taking into account pre-retinal absorption as well as nominal cone
% axial density. But not taking into account any pigment bleaching.
% source
% 'Boynton' -- Boynton and Kaiser, Human Color Vision, 2nd edition,
% pp. 211 and following. [Default.]
%
% 05/23/14 dhb Wrote it.
% 05/26/14 dhb Clean up.
% 06/02/14 dhb Take isomerizations number based on 2:1 L:M assumed ratio.
%% Specify receptor type
if (nargin < 2 || isempty(receptortype))
receptortype = 'cones';
end
%% Specify units
if (nargin < 2 || isempty(units))
units = 'trolands';
end
%% Specify source
if (nargin < 3 || isempty(source))
source = 'Boynton';
end
%% Do it
switch (receptortype)
case 'cones'
switch (source)
case 'Boynton'
switch (units)
case 'trolands'
Izero = 10^4.3;
case 'isomerizations'
Izero = 10^6.4;
otherwise
error('Unkown input units specified');
end
fractionBleached = (intensity./(intensity + Izero));
otherwise
error('Unknown source specified');
end
otherwise
error('Unknown receptor type specified');
end
%% Test code.
%
% If you run the lines below you should get a plot that
% looks like Figure 6.3 (p. 212) in Boyton and Kaiser
% (red curve in plot with blue overlay) plus a shifted
% copy in blue.
TEST = 0;
if (TEST)
trolands = logspace(0,7,1000);
isomerizations = 128*trolands;
fractionBleached = ComputePhotopigmentBleaching(trolands,'cones','trolands','Boynton');
fractionBleached1 = ComputePhotopigmentBleaching(isomerizations,'cones','isomerizations','Boynton');
figure; clf; hold on;
plot(log10(trolands),fractionBleached,'r','LineWidth',6);
plot(log10(isomerizations),fractionBleached1,'b','LineWidth',6);
plot(log10(trolands),fractionBleached1,'b','LineWidth',2);
end
end
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