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Building Environments > Lighting > Flashcards

Lighting Flashcards

1
Q

What factor affects the amount of electro-magnetic radiation that a body emits?

A

It’s absolute temperature (K).

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2
Q

What is our maximum relative sensitivity?

A

555nm (Green)

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3
Q

What is the light spectra?

A
  • 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.
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4
Q

What is the difference between rods and cones?

A

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.
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5
Q

What is a solid angle?

A
  • 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.
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6
Q

What is the derivation for the solid angle?

A
  1. The surface area of a sphere is 4πr2.
  2. Therefore, a sphere subtends to 4π steradians.
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7
Q

What is luminous intensity (I)?

A
  • 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.
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8
Q

What is luminous flux (Φv)?

A
  • 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.
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9
Q

What is the equation for luminous flux (Φv)?

A

Luminous flux (Φv) = Luminous intensity (I) x Solid angle (ω)

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10
Q

What is illuminance (E)?

A
  • 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.
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11
Q

What is luminous efficacy?

A

Luminous efficacy is the light sensitivity of the human eye as a function of frequency.

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12
Q

What is the luminous efficacy for your scotopic and photopic vision?

A
  • 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.
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13
Q

What is luminous efficiency?

A
  • 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%.
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14
Q

What is the luminous efficiency of radiation?

A
  • The luminous efficiency of radiation is the relative degree to which radiation evokes a visual response.
  • Symbol = Vλ
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15
Q

How do you carry out the visual sensitivity experiment?

A
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16
Q

What are sky models?

A
  • 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.
17
Q

Why are standard skies used?

A
  • 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.
18
Q

How can the validity of standard skies be increased?

A

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.

19
Q

What are the equations for standard skies?

A
20
Q

What is a design sky?

A
  • 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.
21
Q

How do you check if enough daylight will reach the external wall of a building?

A

You can use the 25o test.

22
Q

How can we ensure that any future neighbouring buildings will have enough access to daylight when building a new site?

A

Use the 43o test.

23
Q

What is the daylight factor?

A
  • 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.
24
Q

What is the equation for daylight factor?

A
25
Q

What is the equation for average daylight factor (over all surfaces)?

A
26
Q

What is the equation for average daylight factor (on the working plane)?

A
27
Q

What is a rough rule for ensuring a daylight factor of 2%?

A

A floor areas 25x bigger than the glazed area will give a daylight factor of 2%.

28
Q

What is the equation for diffuse light transmittance?

A
29
Q

What is the equation for the average reflectance of interior surfaces?

A
30
Q

What is the daylight distribution within a room?

A

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.

31
Q

What rules can be followed to ensure a whole room is lit well enough?

A
  • 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.
32
Q

What equality should be satisfied in a room with windows in only one wall, to ensure it is well-lit?

A
33
Q

What is uniformity?

A
  • 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.
34
Q

What is the equation for uniformity?

A
35
Q

What effect does window position have on the daylight factor?

A
36
Q

What are the different kinds of daylight-linked controls?

A
  • Manual
  • Timed switch-off
  • Photoelectric switching on/off
  • Photoelectric dimming
37
Q

How does photoelectric dimming work?

A
  • 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%.

Building Environments (6 decks)

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