Lighting Flashcards
What factor affects the amount of electro-magnetic radiation that a body emits?
It’s absolute temperature (K).
What is our maximum relative sensitivity?
555nm (Green)
What is the light spectra?
- Light consisting of high energy photons has a short wavelength, and light consisting of low energy photons has a long wavelength.
- It is the frequency/wavelength of radiation which determines the colour it appears.
- Our visible range is 380-780nm.
What is the difference between rods and cones?
Rods:
- Responsible for seeing in black and white (scotopic vision).
- Very sensitive (1000x more sensitive than cones).
- Slow to react to changes in light.
Cones:
- Responsible for seeing in colour (photopic vision).
- Not very sensitive.
- Quickly react to changes in light.
What is a solid angle?
- The solid angle is the 3D angle produced by a point which radiates light.
- The units for solid angle are the steradian (ω).
- One steradian (ω) is the solid angle which produces a surface are equal to r2.
What is the derivation for the solid angle?
- The surface area of a sphere is 4πr2.
- Therefore, a sphere subtends to 4π steradians.
What is luminous intensity (I)?
- Luminous intensity (I) is the power emitted by a light source in a particular direction.
- Unit = candela (cd)
- Symbol = I
- Luminous intensity (I) is based on the luminous function to account for the varying sensitivity of the human eye to different wavelengths.
What is luminous flux (Φv)?
- The luminous flux is the total power of radiation which is perceived as light by the human eye.
- Unit = lumen (lm)
- Symbol = Φv (v indicates visible light)
- The luminous flux is the sum of the powers of all wavelength in the visible band.
What is the equation for luminous flux (Φv)?
Luminous flux (Φv) = Luminous intensity (I) x Solid angle (ω)
What is illuminance (E)?
- Illuminance is the density of luminous flux reaching a surface.
- Unit = Lux (lx) (lm/m2)
- Symbol = E
- Illuminance is the lighting effect which occurs when luminous flux falls on a surface and illuminates it.
What is luminous efficacy?
Luminous efficacy is the light sensitivity of the human eye as a function of frequency.
What is the luminous efficacy for your scotopic and photopic vision?
- During the evenings (scotopic vision), the peak sensitivity is shifted from 555nm to 507nm.
- At 555nm both photopic and scotopic conversions are the same - 683lm/W.
- Because of the shift towards the UV end of the visible light spectrum, the scotopic vision has little response to light beyond 600nm.
What is luminous efficiency?
- Luminous efficiency is the proportion of the maximum possible efficacy (683lm/W) which a source emits.
- A 100% efficient source would be monochromatic 555nm (green laser).
- The maximum efficiency of a practical lamp is about 40%.
What is the luminous efficiency of radiation?
- The luminous efficiency of radiation is the relative degree to which radiation evokes a visual response.
- Symbol = Vλ
How do you carry out the visual sensitivity experiment?
What are sky models?
- Sky models are mathematical models of ideal luminous distributions under different sky conditions.
- They generate continuous sky luminance patterns.
- Daylight factors are calculated for standard skies.
Why are standard skies used?
- To simplify a complex situation.
- Approximate skies that produce the lowest daylight levels within buildings.
- Provide standard (worst-case) conditions for the comparisons of different designs.
How can the validity of standard skies be increased?
Validity can be increased by applying correction factors that take account of long term variations in the luminance of skies in different quadrants of the sky.
What are the equations for standard skies?
What is a design sky?
- The ‘design sky’ is the illuminance level that is exceeded 85% of the time during the hours of 9am to 5pm throughout the year.
- It is specific to a particular location (latitude).
- Using its value we can easily convert a daylight factor into and estimate of interior illuminance value - by multiplying the daylight factor by the design sky value.
How do you check if enough daylight will reach the external wall of a building?
You can use the 25o test.
How can we ensure that any future neighbouring buildings will have enough access to daylight when building a new site?
Use the 43o test.
What is the daylight factor?
- The average daylight factor is used as a measure of the general illumination from skylight (not including sunlight).
- All rooms in buildings should be predominantly daylight (not artificial) and therefore have an average daylight factor of at least 2%.
- If the average daylight factor is greater than 5%, no artificial lighting is usually needed.
- However, if the average daylight factor is too great, visual discomfort may occur.
What is the equation for daylight factor?
What is the equation for average daylight factor (over all surfaces)?
What is the equation for average daylight factor (on the working plane)?
What is a rough rule for ensuring a daylight factor of 2%?
A floor areas 25x bigger than the glazed area will give a daylight factor of 2%.
What is the equation for diffuse light transmittance?
What is the equation for the average reflectance of interior surfaces?
What is the daylight distribution within a room?
Even if the average daylight factor is high enough, the room may be too long to be day-lit successfully, and so have areas below an acceptable level.
What rules can be followed to ensure a whole room is lit well enough?
- Depth cannot be significantly greater than width.
- Depth can be no more than 2x or 3x the height of the window above the floor.
- The surfaces at the back of the room must be light in colour.
What equality should be satisfied in a room with windows in only one wall, to ensure it is well-lit?
What is uniformity?
- Uniformity is a measure of the range of daylight factors across a room, showing how daylight varies across a space.
- Generally, you want to limit this variability across the working plane.
- A result of 0.25-0.3 is considered acceptable.
What is the equation for uniformity?
What effect does window position have on the daylight factor?
What are the different kinds of daylight-linked controls?
- Manual
- Timed switch-off
- Photoelectric switching on/off
- Photoelectric dimming
How does photoelectric dimming work?
- Both internally and externally mounted photocells monitor the brightness allowing artificial light levels to be adjusted accordingly.
- Photoelectric dimming is only appropriate for areas where the daylight factor is greater than 2%.
