Week 5: Photometry, Photometric Quantities & Instrument

Question 1

Describe what is radiometry

Answer 1

• Measurement of light waves in any of the electromagnetic spectrum
• Electromagnetic spectrum describe all of the kinds of lights: Gamma rays, X-rays, Ultra-violet, visible light, Infra-red radiation, microwave radiation

Question 2

Quantities & Units for Radiometric and Photometry

Answer 2

refer to table page 1

Question 3

Briefly describe photometry in general?

Answer 3

• Photometry is same as radiometry but the range is limited by spectral response of the eye

Question 4

How do our Eyes Responding with Light Wave?

Answer 4

• In human eyes, retina is the place where it has two types of photoreceptors: Rod & Cone cell

Question 5

Explain rod & cone cells?

Answer 5

• Rod cells = more sensitive with low level of light (Night vision)
- It does not play a role in colour vision

• Cone cells = respond differently to light: Blue-sensing cones (short wavelength – 380 to 540 nm), Green-sensing cones (medium wavelength – 440 to 670 nm), Red-sensing cones (long wavelength – 550 to 690 nm)

• Rod & cone cells in retina are activated by light and sends signals to brain through optic nerve

Question 6

Describe Luminous Flux (Luminous Power)

Answer 6

• The measure of the perceived power of light
• Total amount of light emitted by the source
• SI unit = lumen (lm)
• Formula: Φv = 683 ∫Φ (𝜆)𝑉(𝜆)𝑑𝜆
  Where; 𝑉 (𝜆) is photopic response curve. 1 watt at 555nm = 683 lumen

Question 7

Describe Luminous Intensity

Answer 7

• The power from light source to particular direction (per unit solid angle)
• SI unit = candela (cd)
• Formula: Iv = lm/sr

Question 8

Describe Illuminance

Answer 8

• Power of light source on the surface
• SI unit = lux (lx)
• Formula Ev = lm/m2

Question 9

Describe Inverse Square Law

Answer 9

• Distance from light source to surface affects illuminance
• Formula: E = 1/r2

Question 10

Describe Luminance (Nit)

Answer 10

• The quantity value how much luminous intensity can transmit or reflect to human eye
• SI unit = candela per square meter (cd/ 𝑚2)
• Formula: Lv = cd/m2 = lm/sr*m2

Question 11

What is the difference between illuminance and luminance?

Answer 11

• Illuminance = the amount of light from luminance flux impacting a surface per unit of area
• Luminance = the amount of light coming from light source in a given direction

Question 12

What is the difference between luminance and brightness?

Answer 12

• Luminance is physical value: amount of light (energy) coming from region of space
• Brightness is perceptual value: perceived amount of light coming from a source, referred as self-luminous –> subjective
• Steven’s power law

Question 13

Describe Photopic Vision

Answer 13

• Maximum visual effect at 555nm (yellow-green)
• Cone cells –> perceive colour & light adapted
• Formula: Φ𝑣 = 683 ∫ Φ(𝜆) 𝑉(𝜆)𝑑𝜆  1 watt at 555nm = 683 lumen

Question 14

Describe Scotopic Vision

Answer 14

• Maximum effect at 507nm (blue-green)
• Rod cells work –> perceive shapes & dark adapted
• Formula: Φ𝑣 = 1700 ∫ Φ(𝜆)𝑉s(𝜆)𝑑𝜆  1 watt at 507nm = 1700 lumen
  Where; 𝑉𝑠 (𝜆) is the SCOTOPIC response curve

Question 15

Describe Mesopic Vision

Answer 15

• Combination of photopic & scotopic (low light conditions)

Question 16

Describe solid angles

Answer 16

• Luminous intensity = cd=lm/sr
  Where; sr is steradian (solid angles)  3-dimension angular volume defined analogously to the definition of a plane angle in two dimensions
• Sphere= 4𝜋sr

Question 17

Briefly describe Goniophotometer instrument

Answer 17

• Instrument used for measuring the light emitted from an object at different angle
• By rotating, goniophotometer measures luminous flux, intensity distribution and illuminance distribution of a light source

Question 18

Briefly describe Sphere Photometers instrument

Answer 18

• Integrating sphere is used to measure luminous flux
• Sphere photometer measures the total light radiated in all directions from a lamp

Question 19

Briefly describe Lux Meter (Illuminance) instrument

Answer 19

• Lux meter is used to measure illuminance
• Can measure up to 400,000 lux (depending on instrument) – handheld instrument

Question 20

Briefly describe Luminance Meterinstrument

Answer 20

• Using optical lens system, luminance (cd/m2 )
• Designed sensor with a spectral response where match spectral luminous efficiency function the human eye

Question 21

What are the estimated values for the lux in different days?

Answer 21

refer to table on page 3