There is quite a bit of information in this graph. The purple, dashed line shows the course of rhodopsin regeneration after it was completely bleached by a very bright light. Time represents the amount of time in darkness after bleaching. The ordinate for the dashed, purple line is on the right and you can see that at time 0 there is almost 0% unbleached photopigment (rhodopsin). This purple curve represents the function for both a normal sighted person and a person that has only rod receptors (rod monochromat). This data was collected with a retinal densitometer.
The squiggly green line represents the psychophysical threshold function for the rod monochromat and the solid red line is similar to the psychophysical cone threshold function for a normal sighted person and the red dotted curve the rod function.. The threshold curves should be view with respect to the left-hand ordinate.
The early stage of the psychophysical function for rod threshold is not seen for a normally sighted person because the cone receptors are more sensitive than the rods. You can see, in this graph and also in the dark adaptation graph that the cone function reaches a plateau after about 7 minutes in the dark. Rod vision takes over when it becomes more sensitive. The significance of the graph on the right is that because the same retinal densitometry function for rods is found with a normal and rod monochromatic observer we can confidently assume that the extrapolation of the early stage of rod sensitivity shown in the dark adaptation curve is correct.
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