Illuminance and Luminance

Question 1

How can you use the reverse contrast snellen chart?

Answer 1

Black chart with white letters. More contrast so we don’t “dazzle” our cone system.

Could use this method with cataract patients and pinhole to give indication of vision after surgery.

Question 2

How to calculate illumination if and if the surface is perpendicular to the light source and if it isn’t?

Answer 2

If it is perpendicular: E= (l/d^2)
If it is not perpendicular, there will be a reduction in illumination. E= (I/d^2)(cos theta)

E= illumination of the surface
I= luminous intensity of the source
d= distance in m

Question 3

Factors that can affect the amount of light that reaches the retina?

Answer 3

Pupil size
Corneal, vitreal and lens transparency
External factors (dirty windshield and lenses)

Question 4

Troland

Answer 4

A unit for retinal illumination. Takes pupil area into account in mm^2.
T=LA, A= (pi)(r^2) r in mm

Troland= luminance of the surface being viewed (Cd/sq m) (pupil area in mm^2)

Question 5

Abney’s law of additivity

Answer 5

If we combine lights of different wavelengths together (add their luminance) the mixture’s brightness should exactly match that of a monochromatic light having the same color and luminance.

The mixtures brightness should be exactly the same as a light of either one of the component wavelengths would be if it were increased to the same luminance as the mixture.

Question 6

When does Abney’s law work best?

Answer 6

When the lights in the mixture are similar in wavelength.
And when you compare brightness using the Heterochromatic flicker photometry (HFP) or the minimally distinct border criterion (MDB)

Question 7

When is Abney’s law most likely to fail?

Answer 7

If lights in the mixture are different in wavelength

Brightness comparison using heterochromatic brightness matching

Question 8

Difference between measurements of light energy and measurements of brightness.

Answer 8

Radiometric measurements only represent the physical properties of light.
Brightness relies on the observers perception so it is subjective. Depends on optical, neural, and psychological properties of the observer in addition to the lights physical properties.

Question 9

Cosine diffusers or lambert surfaces

Answer 9

Perfectly diffusing, matte, surfaces.
The luminance measured from the surface is always the same, no matter where it is taken from. This means brightness is also perceived the same.

The illumination falling onto one of these surfaces will reflect at a factor between 0 and 1. This can be used to solve for luminance.

L= rE
L= luminance
r= reflection factor between 0 and 1 
E= illuminance

Question 10

Units for illuminance

Answer 10

Lumens/m^2 = lux

older units are called foot candles

Question 11

Difference between radian and steradian

Answer 11

Radian cuts out of a flat surface

Steradian cuts out of a sphere

Question 12

The inverse square law states that

Answer 12

The number of lumens from a POINT SOURCE falling on a surface decreases with the square of the distance between the light source and the surface IF the surface is normal (perpendicular) to the light source.

E= I/d^2
E= illumination of the surface
I= luminous intensity of the source 
d= distance between surface and source

Question 13

Cosine law of illumination

Answer 13

If the surface is nor normal to the point source, there is a reduction in illumination- proportional to the cosine of the angle of tilt of the surface

E= I/d^2 (cos angle)

Question 14

Brightness depends on __, __, and __ properties of the observer in addition to the light’s physical properties.

Answer 14

Optical, neural and psychological