VOP - The Light Sense I - Topic 2 Flashcards
Which type of light level do cones operate at? and what is the luminance level?
Cones operate under photopic light levels – a luminance level of more that about 10 cd/m2.
Which type of light level do rod operate at? and what is the luminance level?
Rods operate under dark adapted conditions known as scotopic (less than about 10-3 cd/m2).
Which lighting conditions do both rods and cones operate at?
Mesopic lighting conditions
-between 10-3 cd/m2 & 10 cd/ m2
Define Spectral Sensitivity.
Sensitivity refers to the ability of the photoreceptor to detect light
How is the change in sensitivity described for cones?
Vλ (‘vee lambda’) curve (for cones)
How is the change in sensitivity described for rods?
V’λ (‘vee prime lambda’) for rods
Do rods and cones respond differently to different wavelengths of light?
Yes
What are the differences in sensitivity to wavelengths of light between photopic(cones) and scotopic(rods) spectral sensitivity functions?
- Rods are more sensitive to lower wavelengths of light
- Cones are more sensitive to very long wavelengths of light.
What type of scale is used to measure sensitivity and why?
A log scale,
because of the range of sensitiivity of the eye.
How can you compare the Vλ and V’λ curves?
- compare by ‘normalising’ so that the peak for each equals 1 for luminous efficiency.
- Peak sensitivity is about ~510 nm for rods and 555 nm for cones
What is the purkinje shift?
As overall luminance decreases from photopic to mesopic levels (light to dark), there is an enhancement to shorter wavelengths (blue) , as the peak of the curve shifts from cone Vλ to rod V’λ function.
Rods & Cones respond differently to different wavelengths of light, as a result, what happens to the Vλ curve?
There will be a shift in peak sensitivity from photopic to scotopic/mesopic light levels.
What is sensitivity defined as in terms of threshold?
Sensitivity= 1/threshold.
What is dark adaptation?
- When we move from a well lit area (e.g. outside sunlight) into a dark area, our eyes take some time to ‘adapt’.
- The more time spent in the dark, the more the sensitivity of the eye increases
What photopigments do rods and cones have?
rods- rhodopsin
cones- red(long), green(medium) or blue(short wavelength)
Which chemical reaction do photopigments undergo upon absoprtion of light?
Bleaching
What is measured when you measure dark adaptation?
- we measure the ‘recovery’ of the sensitivity of cones and rods over time.
- Threshold for detection was measured.
Define absolute threshold?
The absolute threshold is the lowest light energy level of a stimulus that can be perceived.
How could knowledge of dark adaptation function be useful in a clinical application
Used for Patients that suffer from symptoms of night blindness.
- There will be no rod function, so no rod/cone break
- Only cone function.
- So curve plateau after short time
At absolute threshold, how many quanta are absorbed by rod photopigments?
5-14 quanta
What is a quantum?
the smallest possible discrete unit of any physical property, such as energy or matter
How many quanta is needed to activate a rod?
A single quantum is sufficient to activate a rod.
-a single activated rod is not sufficient for see
What is the peak sensitivity for rods and cones?
Rods ~ 510nm
Cones ~ 555nm
What is the luminance level for photopic conditions?
Photopic > 10cd/m2
What is the luminance level for Mesopic conditions?
Mesopic 0.001 – 10 cd/m2
What is the luminance level for scotopic conditons?
Scotopic < 0.001 cd/m2
What is the number of quanta required at the threshold for seeing?
The number of quanta at the threshold for seeing = number of rhodopsin molecules required = number of neural impulses
What’s Threshold intensity defined as?
defined as the energy when the stimulus was seen 6 out 10 times correctl
What is pooling?
– The signal generated in the rod cell passes through different neural elements in the retina to the retinal ganglion cells.
–The ganglion cell combines the input from a number of rod cells over a relatively large area
What does pooling allow?
It allows the scotopic rod system to have very good sensitivity to detect light but poorer or lesser ability to know how many individual lights are visible (i.e. poor spatial resolution)
