Photometry

Question 1

Define:

Illuminance at a point on a surface

Answer 1

Illuminance at a point of surface is the amount of total luminous flux incident on a surface, per unit area. (lux)

Question 2

Laws of photometry

Answer 2

Law 1: The Inverse Square Law of Illuminance states that the illuminance at a point on a surface is
inversely proportional to the square of the distance between the point and the source

Law 2: The Cosine Law states that if the normal to an illuminated surface is at an angle (θ) to the
direction of the incident light, the illuminance is proportional to the cosine of the angle (cosθ)

Question 3

Laws of photometry

Answer 3

Law 1: The Inverse Square Law of Illuminance states that the illuminance at a point on a surface is
inversely proportional to the square of the distance between the point and the source

Law 2: The Cosine Law states that if the normal to an illuminated surface is at an angle (θ) to the
direction of the incident light, the illuminance is proportional to the cosine of the angle (cosθ)

Question 4

Formula for:

Directly below a source

Below at an angle

Answer 4

Directly below - E = 1\d2.

Below at an angle - E = 1\d2 x Cosθ

Question 5

Define:

Transmittance of a transparent body

Answer 5

The amount of light that passes through the transparent medium