VOP - The Light Sense I - Topic 2

Question 1

Which type of light level do cones operate at? and what is the luminance level?

Answer 1

Cones operate under photopic light levels – a luminance level of more that about 10 cd/m2.

Question 2

Which type of light level do rod operate at? and what is the luminance level?

Answer 2

Rods operate under dark adapted conditions known as scotopic (less than about 10-3 cd/m2).

Question 3

Which lighting conditions do both rods and cones operate at?

Answer 3

Mesopic lighting conditions

-between 10-3 cd/m2 & 10 cd/ m2

Question 4

Define Spectral Sensitivity.

Answer 4

Sensitivity refers to the ability of the photoreceptor to detect light

Question 5

How is the change in sensitivity described for cones?

Answer 5

Vλ (‘vee lambda’) curve (for cones)

Question 6

How is the change in sensitivity described for rods?

Answer 6

V’λ (‘vee prime lambda’) for rods

Question 7

Do rods and cones respond differently to different wavelengths of light?

Answer 7

Yes

Question 8

What are the differences in sensitivity to wavelengths of light between photopic(cones) and scotopic(rods) spectral sensitivity functions?

Answer 8

• Rods are more sensitive to lower wavelengths of light

- Cones are more sensitive to very long wavelengths of light.

Question 9

What type of scale is used to measure sensitivity and why?

Answer 9

A log scale,

because of the range of sensitiivity of the eye.

Question 10

How can you compare the Vλ and V’λ curves?

Answer 10

• 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

Question 11

What is the purkinje shift?

Answer 11

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.

Question 12

Rods & Cones respond differently to different wavelengths of light, as a result, what happens to the Vλ curve?

Answer 12

There will be a shift in peak sensitivity from photopic to scotopic/mesopic light levels.

Question 13

What is sensitivity defined as in terms of threshold?

Answer 13

Sensitivity= 1/threshold.

Question 14

What is dark adaptation?

Answer 14

• 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

Question 15

What photopigments do rods and cones have?

Answer 15

rods- rhodopsin

cones- red(long), green(medium) or blue(short wavelength)

Question 16

Which chemical reaction do photopigments undergo upon absoprtion of light?

Answer 16

Bleaching

Question 17

What is measured when you measure dark adaptation?

Answer 17

• we measure the ‘recovery’ of the sensitivity of cones and rods over time.
• Threshold for detection was measured.

Question 18

Define absolute threshold?

Answer 18

The absolute threshold is the lowest light energy level of a stimulus that can be perceived.

Question 19

How could knowledge of dark adaptation function be useful in a clinical application

Answer 19

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

Question 20

At absolute threshold, how many quanta are absorbed by rod photopigments?

Answer 20

5-14 quanta

Question 21

What is a quantum?

Answer 21

the smallest possible discrete unit of any physical property, such as energy or matter

Question 22

How many quanta is needed to activate a rod?

Answer 22

A single quantum is sufficient to activate a rod.

-a single activated rod is not sufficient for see

Question 23

What is the peak sensitivity for rods and cones?

Answer 23

Rods ~ 510nm

Cones ~ 555nm

Question 24

What is the luminance level for photopic conditions?

Answer 24

Photopic > 10cd/m2

Question 25

What is the luminance level for Mesopic conditions?

Answer 25

Mesopic 0.001 – 10 cd/m2

Question 26

What is the luminance level for scotopic conditons?

Answer 26

Scotopic < 0.001 cd/m2

Question 27

What is the number of quanta required at the threshold for seeing?

Answer 27

The number of quanta at the threshold for seeing = number of rhodopsin molecules required = number of neural impulses

Question 28

What’s Threshold intensity defined as?

Answer 28

defined as the energy when the stimulus was seen 6 out 10 times correctl

Question 29

What is pooling?

Answer 29

– 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

Question 30

What does pooling allow?

Answer 30

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)