Lesson 1 Flashcards
What is relative spectoral sensitivity?
Our eyes are more sensitive to some wavelengths than others. In particular the wavelengths at the middle of the visible light spectrum. Relative spectoral sensitivity is 1 for green and a fraction of 1 for all other colours in the visible light spectrum.
The output in lux of a light is given by the power is watts x 683 x the spectoral sensitivity of a particular wavelength.
Explain total lux of different wavelengths of light.
The power of each wavelength in watts x the spectoral sensitivity of each wavelength x 683. The total light output is the sum of all wavelenghts calculated using the above formula.
Explain the difference between:
Luminous flux
Luminous intensity
Illuminance
Luminance
Luminous flux: “Is light emmitted by a source or received by a surface”
Luminous intensity (I): “Luminouse flux emmitted per solid angle in a given direction.
Illuminance (E): “The luminous flux density at a point on a surface”
Luminance (L): “The luminous intensity of light emitted or reflected in a given direction from an element of the surface, divided by the area of the element projected in the same direction”
Explain the relationship between luminous intensity of a source, distance and angle of a reflecting surface. The solid angle is equal to the area divided by the distance squared from the light source. The luminous flux incident on the area is equal to the luminouse itensity divided by the distance squared. If the receiving surface is at an angle then the luminouse intensity is reduce in proportion to the cosine of the angle.
As the distance between the reflecting object and the source increases then the light will be diluted over a larger area. The flux that hits a perpendicular object will be equal to the Area receiving/distance squared from the source. The units are steradians.
If the receiving area is at an angle to the light source then the lux will be reduced in proportion to the cosine of the ang.e.
Explain what a candela is
The luminouse intensity in a given direction of a source that emitts 1 lux of (555nM) light (peak spectoral sensitivity) at an angle of 1 steradian (or 57.295 degrees).
Explain illuminance
Illuminance of a surface (measured in lux) is the flux per unit area. In a room the luminance is going to vary from cm to cm depending on the distance and the angle from the source. This can only practically be calculated using lighting design software.
Explain the concept of glare
Discomfort glare causes discomfort with neccessary imparing the appearance of objects. Disability glare impares the appearance of objects without neccessarily causing discomfort. Discomfort glare is the main concern in indoor lighting design.
Explain the unified glare rating and how is it calculated
Glare varies with the position and viewing angle of a person in a particular room. Manufacturers unified glare ratings assume a worst case scenario within a particular room. UGR ratings range from 5 to 40 with 10 being very little glare.
