Light Luminaires

A luminaire by definition is a complete lighting unit. It consists of a lamp or lamps together with the parts designed to electrically operate and control the lamp. It is used

  • To distribute light
  • To control the light
  • To position light
  • To protect the lamps physically
  • To connect the lamps to the power source.

For a given interior lighting application designer needs to know about a luminaire. It is because; a perfect luminaire selection gives perfect appearance and best utilization of luminous flux.

Luminaire Flux Distribution

We can use a gonio photometer to obtain the intensity distribution curve of a lamp or luminaire. Gonio means angles. Gonio photometer is an instrument that is used to measure the flux distribution of a luminaire and intensity diagram. The overall lumens emitted and the lumens emitted in certain angular zones can be calculated easily with this goniophotometer.
To choose a luminaire, a designer should be careful about

  • The percentage of the total lumen output that is directed upward toward the ceiling,
  • The percentage that is directed downward towards the work plane (an imaginary horizontal plane usually 30 in above the floor where the desk tops, etc. are located),
  • The directivity of the downward component. The directivity is generally quantified in terms of the percentage of the luminaire lumens in the 0 to 40 degree zone.

The CIE has recommended the interior luminaires based on a six-category classification system. This classification is related to the upward and downward lumens. It is given in table below.

This six-category classification system shows the luminaires according to the percent of the luminaire output emitted above and below the horizontal.
In the consideration the shape of the upward and downward distribution is not taken. But exceptionally, in case of general diffuse and direct lighting the shape of the upward and downward distribution is taken into account, where the latter classification implies very little luminous flux at angles near the horizontal.

Luminous Efficacy

Luminous Efficacy is defined as the ratio of the lumens emitted by the luminaire to the lumens emitted by the lamps.
The lower this number the more luminous flux is being trapped and absorbed by the luminaire.
If the efficacy is 70%, then it indicates that luminaire design is very poor. The sum of the percent upward and downward luminaire lumens always should be equal to 100, regardless of the luminaire efficacy.

Luminaire Luminance

We know that bare lamps create too much glare as they are too bright. For this reason the luminaire must be capable to filter and shield the bare lamp from the viewers to provide glare free environment.
The customer must get the luminaire maximum and average luminance data provided by the luminaire manufacturer along with the luminaire. At a particular viewing angle, maximum luminance is a measured quantity. Generally several values of maximum luminance are given within the 45 to 90 degree zone. In this zone direct glare is likely to be bothersome, for both cross wise (perpendicular to the luminaire) and length wise (along the luminaire) viewing. The direct and indirect glare zones are shown below.
luminaire luminance
Average luminance is a calculated quantity in a particular viewing direction. Average luminance is obtained by dividing intensity of the luminaire in the given direction by projected area of the lighted portion of the luminaire in that direction.
Four basic types of luminaires are shown in the figure below where

  • Luminaire (a) is surface mounted and has dark side panels.
  • Luminaire (b) is suspended but has no upward component. It does have luminous side panels.
  • Luminaire (c) is also suspended but has an upward component and thus its effective lighted width is greater than that of the luminaire itself.
  • Luminaire (d) is like luminaire (c) except that is has dark side panels. Thus its effective lighted width is split into two parts, a luminaire portion and a ceiling portion.

projected areas of basic luminaires
Let us assume each luminaire in the above figure has a length l and a projected lighted width wp in the direction of an observer who is viewing the luminaires cross wise.
I is the average intensity of the luminaire in the direction of the observer, then the average luminance can be presented in this direction as

Here, Apo is the crosswise projected area.
As per the figure of luminaire viewing, shown below, the viewer is at point O. Parallel rows of the luminaires, one of which is indicated by the line from S0 to S, are located on the ceiling. Consider a single luminaire centered at point S0 and having a projected area Avo in the direction of O. This yields the luminance given by above equation.
Now let the luminaire slides down the row from S0 to S. The width of Apo does not change, but its length does. The new projected area in the direction of O is

It is possible to express Φ in terms of the polar angles at S0 and S. these are given the label Ɵ0 and Ɵ and we have

Now we can get,

Assuming the luminaire at S has an average intensity l in the direction of O, the average luminance of that luminaire as seen at O is

It is usually considered that direct glare will not be a problem if within the 45 to 90o glare zone the ratio of the maximum to average luminaire luminance does not exceed 5:1, for both cross wise and length wise viewing, and if the maximum luminaire luminance is less than 5 cd/in2 at 45o, 2.5 cd/in2 at 65o and 1 cd/in2 at 85o.
general case of luminaires viewing

Luminaire Light Control

The bare lamp lighting does not distribute the luminous flux efficiently and effectively. Bare lamp lighting is the source of glare.
There are many methods to control the light and glare in contemporary luminaire. Reflectors, Diffuser and Baffles are widely used to control the light along with the contemporary luminaires.
Diffuser

  • Diffusers are clear prismatic lenses, flat translucent sheets or combination of either of glass or plastic.
  • Diffusers are used to reduce luminance within the 45 to 900 glare zone on the bottom and sides of luminaires to redirect or scatter the light.
  • Clear prismatic lenses are used to follow the principle of refraction to produce desired downward intensity distribution.
  • A variety of patterns are used like series of conical or pyramidal shaped prisms with a density of 25 to 64 per square inch.
  • Translucent diffusers always scatter the light in all downward directions. Rather, they never direct the light to particular zones.
  • Transmittance is a function of pigment density and thickness of the diffuser.
  • Transmittance of 1/8 inch sheet is 45 to 75 %.
  • Considering the same pigment density, transmittance will drop about 20 percent if the thickness becomes double.
  • Actually Diffuser has low absorption capability.
  • Most of the non-transmitted luminous flux gets reflected back into the luminaire. Again, much of them return to the diffuser, increasing transmission.

Baffles and Louvers

  • A baffle is a single shielding element often placed parallel to and between lamps in two-lamp luminaire.
  • Usually it is V shaped.
  • A louver is a group of baffles in an egg crate arrangement.
  • The louvers may be either straight or parabolic.
  • The modern louvers control the directivity of the downward lumens and low luminance in the field of view.
  • The louvers come in a various sizes and shapes, materials and finishes.
  • The louvers finish can be specular silver, gold or bronze, translucent white or opaque black.

The shielding angle of a baffle or louver is the angle between the horizontal and that line of sight for which all objects above are concealed. It is shown as Ɵg in the below figure. The actual lamp shielding angle may be somewhat greater than Ɵg, if there is space between the lamps and tops of the louvers.
luminaires
In areas where low ceiling brightness is required, such as with visual display units, small hexagonal or square cell louvers are often used. The cells are typically ½ inch deep and ½ to ¾ inch wide. Each vertical cell is parabolic and the shielding angle is typically 45 degree. A disadvantage is that the resulting luminaire efficiency is low often less than 50%.
Reflectors

  • Reflectors are of two types, specular or semispecular, and diffuse.
  • They are mounted above the lamps to direct the upward component of the luminous flux.
  • Basic specular contours are circular, parabolic, elliptical or combination thereof. General specular contour may be tailor made to produce a particular overall luminous intensity distribution and are frequently designed graphically, rather than mathematically.
  • They are made of aluminum which is etched, polished, brushed, plated or anodized to produce the reflecting surface. The anodizing procedure is an electrochemical process which deposits a uniform permanent and high reflectance surface of aluminum oxide on the aluminum.
  • In the fluorescent luminaires Diffuse reflectors commonly used,
  • They serve simply to gather and redirect as much of the upward luminous flux as possible downward to the lens, diffuser or louver.
  • Due to use of reflectors the reflected intensity distribution curve is approximately a cosine distribution,

Other Consideration of a Luminaire

  1. Appearance
  2. Maintenance
  3. Ventilation
  4. Acoustics
  5. Mounting
  6. Radiation
  7. Hazardous Location

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