19 LIGHT – MEASUREMENT, RELATED TERMS AND UNITS

S. Visalakshi Rajeswari

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Introduction

 

Light is that part of the electromagnetic spectrum which will stimulate a response in the receptors of the eye. Its frequency usually expressed as wavelength determines the colour of light and its amplitude determines its intensity. Accommodation from the individual’ s part enables focusing of vision. Hence the need to study lighting in interiors. Especially when activities are carried out indoors it is necessary to provide some sort of artificial illumination. In such circumstances, the designer should be aware of what (lighting) is provided and the satisfaction the ‘user’ derives out of it.

  1. The radiant ene rgy spectrum Vs visible spectrum

Light is visually evaluated radiant energy (electromagnetic), which moves at a constant speed in vacuum. The entire radiant energy spectrum consists of waves of radiant energy that vary in wavelength of a wide range; an array of all rays – cosmic, gamma, UV, infra red, radar, x rays, the visible spectrum, FM, TV- and radio broadcast waves and power transmission

The portion of the radiant energy which is seen as light, identified as the spectrum visible to the human eye ranges from about 380 (400) to 780 (700) mµ (referred to as nanometers or millimicrons). A nanometer (nm) is a unit of wavelength equal to 10 -9 m.

 

Light can thus be thought of as the aspect of radiant energy that is visible. Colour perception is attributed to the varying wavelengths noticeable within the spectrum of visible light.

 

A mixture of most of the wavelengths within this range will give rise to a ‘white light’, but if any wavelengths predominate the light will be coloured. Thus a coloured light can be defined in terms of the intensity and wavelength of its components.

 

There are two sources from which light is received. One is from incandescent bodies (sources which are hot like Sun, bright luminaires, natural or artificial flame etc) and luminescent bodies (cold sources, that is, all the objects in the environment which reflect light).

Sunlight is a simple thermal source and approximately 44 per cent of sunlight energy reaching the ground is visible. The glowing solid particles in a flame are also common thermal light sources. Incandescent bulbs, another example emits only 10 percent of the energy as visible light.

 

3.   Spectral sensitivity of the human eye

 

Except when looking directly at a light source, sensations in the eye are caused by light reflected from the object in view. The human eye does not exhibit equal sensitivity to all wavelengths in the visible spectrum. The eye consists of cone and rod cells, is a known fact, in which the cone cells are of three types. They are found to respond differently across the visible spectrum. The response peaks is somewhere around 555 nm. Naturally, visible light of uniform intensity from twin sources of light are found not to appear equally bright. The eye responds to light as a function of the wavelength. This concept is represented as luminosity function.

 

In this context, the eye responds differently in different situations. It is called photopic vision when it adapts to light, (luminance levels over three candela per sq.m), as scotopic vision when it adapts to dark conditions. Photopic response is the base for Photometry. Hence, the brightness of sources in dim lighting predicted from the data obtained from such a method may not be reliable. Here colours may not be that discernible. Photometry (measurement of light) is a method that assesses this. These units relate to luminous efficacy, a term used to explain raw power by a quantity. It is absolutely necessary to determine/ arrive at sufficient illumination for various tasks in lighting of buildings – both interiors and exteriors.

 

4.    Methods used for measurement of light

 

Light is measured adopting two methods. Radiometry which measures light power individually at all wavelengths and photometry which on the other hand measures light based on wavelength weighted on par with a standardized model of perception of brightness by the human eye are the two sets of units in practice to measure light. For quantifying lighting intended for human use, latter method which measures only the visible light is used. It considers human eye as a detector and gives data in multiple photometric units and measures. There is a factor representing the sensitivity of the eye to particular wavelengths. Photometry allots a weightage to the measured power to individual wavelengths with that factor.

 

Light propagates through three dimensional spaces in many ways. It can spread out, concentrate and/ or reflect off shiny surfaces. Similarly, light consists of different wavelengths. This attribute enables scientists to fundamentally measure different kinds of light in a broad range and also represent them in varied quantities and units. Photometry considers three aspects in measuring light: amount of meaningful light incident on a surface, quantum of light emitted by the source and the wavelengths with their specific colours associated with the light. Therefore measurement of useful light should always consider luminosity function. The unit used is the candela (cd) from which all other units are derived.

 

Light is considered as a form of energy and when it is measured as energy units, it becomes a physical unit. In applications where light is measured based on its brightness, they are explained in terms of subjective units such as candela, lux and the like. It is because of this subjective difference that light is described in multiple units. The word ‘bright’ (usually used to refer to source of light) can be used as an adjective to refer to light in many ways. A bright light is a source:

  • from which emerges a high density light output ( given in units of lumens)
  • from which channelizes and focuses the flux into a very narrow beam ( expressed as candela)
  • focused against a dark background

Naturally, when considering the amount of light required in relation to the task being undertaken it is the brightness of the object which is of importance. This brightness depends on the illumination and on the proportion of the incident light which the objects reflect. Brightness, scientifically speaking is described in two terms: luminosity and luminance.

 

4.1.  Luminosity: defines the subjective characteristics of the visual sensation. They are influenced by various factors not directly related to the light reflected into the eye.

 

4.2. Luminance (brightness): refers to the quantity of reflected light measured photo metrically. It is the amount of light per unit area reflected from or emitted by a surface, per se, that gets channelized within a given solid angle. This factor called luminance is measured in terms of a basic unit known as the Lambert in non – SI unit. The Lambe rt is distinctly stated adopting the symbols – L, la or Lb, proving that it is a non – SI unit of luminance. Contrarily, the SI unit of luminance is candela/  sq. m (cd/m2) and the CGS unit is stilb.

 

5.   Measure ment of light (Photometry)

 

There are many concepts and terms that relate to the intensity of light. Each one has their own units of measurement as explained here individually

 

5.1.  Candela (cd): Formerly called the ‘candle’ is the international unit used to express an estimate of luminous intensity, specifically, its denseness emitted from a source. Luminous intensity is the power of the light source.

 

5.2.  Candle power (cp): It is the unit used to express an estimation of the luminous intensity of a light source as expressed in candelas. It basically is considered a measure of the light produced by a bulb. It is the light measured at level of the bulb, not what falls on a medium.

 

Measurement of the intensity of light is based on the concept of density of Luminous Flux (F), flowing from a standard source of radiant energy, which is arbitrary and is called the “international or standard candle”.

 

5.3.  Luminous Flux: It is defined as the amount of useful light emitted from the source, its light output. More specifically it is the light energy in toto that a source emits across visible wavelengths of light. It relates to the time rate of flow of light measured in Lumens (lm). This refers to the flow of light related to a unit of solid angle. Evidently it is the quantum of light that leaves the lamp.

 

5.4.  Lumen (lm): is considered as the photometric unit of light flow or light output. It is given as the term referring to the unit used for measuring luminous flux. The light emerging from a known point source of one candela (12.57 lumens) falling on an area of one feet square (1 ft2) at a distance of one feet is one lumen. It is also given as equal to one foot candle falling on a surface of one foot square. It is the quantity of continuously generated light. Packs of an incandescent bulb emitting 60 watt and a compact fluorescent (CFL) labeled 15 watt both indicate that they provide 900 lumens each. Therefore this is a measurement of light versus area.

 

5.4.1.    Points to ponder in application

 

  The light output of artificial lights is generally measured in lumens  Common trend is labeling light sources with an output rating in lumens ( both initial and mean ratings)  Is an indicator of the age or length of use of lamps and fixtures (lumen output decreases)

 

5.5.        Lumen rating in lamps

 

5.5.1.   Initial rating: Lamps are rated based on the initial lumen that is when they are new.It is an indicator of the amount of light produced by a lamp, once it has stabilized. With use and age the output tends to decrease.The initial lumen rating for fluorescent tubes and HID (Higher Intensity Discharge) lamps is 100 hours.

  5.5.2.   Mean lumen rating: Lamps are also rated in mean lumens. All lamps are expected to produce an average light output over its rated life. This factor takes into consideration a lamp’s gradual deterioration in performance in the long run of its rated life. One can understand this better this way. As stated earlier, the initial lumen rating for a fluorescent lamp is 100 hours and its mean lumen rating is determined at 40 per cent of rated life. So, after that period, one can expect the lamp’s performance to be low.

 

5.6. Illuminance or luminous intensity : This is what happens when one uses light. It is something created by individuals. It is the quantum of light projected on a specific area and the degree to which objects kept in that surface get illuminated. It indicates the luminous power per area. It highlights the light received by the area. One measure of illuminance is the foot candle (fc), while in SI units it is Lux.

 

5.6.1. Self check exercise

 

Turn on torchlight in a room at night. The flash of light falls where one has focused the torch. Objects falling on the focus of light get lighted, to be precise, they are illuminated. The person using the torch has lighted it up. This is called illuminance. Do not confuse this with the quantum of light emitted by the source.

 

5.7. Foot candle: Considered as the basic unit of illumination, measures the amount of light falling on a surface. One foot candle (fc or lm/ft2) is equal to one lumen distributed evenly over a one square foot area. In other words it is the illuminance received on a specified area or object from a one candela light source placed at a distance of one foot.

 

5.7.1. Learn more

 

Foot candles demonstrate the actual amount of light from a source that falls directly on the object that is meant to be illuminated.

 

It is the most popular explanation, comprehensible by all by its simple definition. For instance, a person buys a lamp where the label assures 100 foot candles of light. It is very clear then that a person sitting one foot away from the source will be able to receive 100 foot candles of light and therefore arrange things which need to be illuminated at that point. The person may increase or decrease the distance and the light source depending on one’s requirement. So, this unit is application- specific and helps in designing lighting based on requirements.

 

Keep a standard candle at a distance of one foot from an angle. Measure the brightness. It will be approximately equal to 10.7639 lux

 

5.7.3.   Use

 

It is the commonly used unit in many fields and purposes like:

 

  •  For controlled light levels to conserve light – sensitive objects like paintings, photographs in museums and galleries
  • To calculate adequate lighting levels of workspaces (indoor and outdoor) in the lighting industry
  • Professional fields of photography, film industry, television, green house horticulture, structural engineering and in suggesting building codes.

5.8.  Illumination: People are not so interested in the total intensity of light given out by the source, but are concerned with the amount of light which falls upon an object. This phenomenon is known as illumination (E). It is defined in terms of lumens per square foot (lm/ ft2). That is, the luminous flux falling upon one square foot of area placed one foot distance from a standard candle, and it is equal to1/ 12.57t h = 1/4of the total luminous flux which is emitted by a standard candle. In C.G.S system, the equivalent unit is popularly called as Lux (which is apparently one lumen (lm) per meter square Popularly referred to as Lux (lx) in C.G.S system it is an equivalent unit given as one lumen per square meter. Lux is an abbreviation for lm/ m2. Stated otherwise, 1 lm /ft2 is equal to 10.764 lux.

 

5.8.1. Learn more

 

A standard candle or any wick is found to radiate light equally in all directions that is in a circle. The output from the wick is not generally made to focus by use of reflecting devices, say a lens or a reflector. Imagine that the light makes a one meter radius sphere around the burning wick. It is a known fact that such a sphere encloses a 12.57 m2 surface area. One lumen is defined as the quantum of energy emerging from 1 m2. Given the circumstances if one tries to reduce the dimensions to represent a radius of one foot, the energy that is reflected from the source can be increased 12.57 times that of the energy which will fall on a 1 m2 area.

(http://hydrogen.physik.uni-wuppertal.de/hyperphysics/hyperphysics/hbase/acoustic/invsqs.html)

 

Lux (lx): Lux is used in photometry, as a measure of the intensity (after analyzing the way the human eye perceives it), of light that strikes or traverses through a surface. It measures the incident light available at a known distance. It is a metric unit of foot candle. One lux is equal to 0.0929 foot candles. So, it is used to be called as a meter – candle.

 

Strictly speaking the lumen measures the rate of flow of the luminous flux from the source, while candela describes the intensity of a light source in a given direction, specifically towards the eye, whether the source is an emitter or a surface which is reflecting incident luminous flux.

 

5.8.2. Inte resting facts

 

Know about a standard rule in lighting

 

If the light falling on an accessory at home is measured one will observe that it is not directly proportional to the distance at which the accessory is placed from the source of light. Scientifically speaking, the lesson learnt is, closer the object is to a light source, say a bulb, brighter one finds the source (bulb) as.

 

5.8.3.    Self check exercise

  • Collect photometric data of lamps from manufacturers or packaging labels
  •  Using simple arithmetic calculate illuminance for a specified area / room.
  • To convert foot candles to Lux, multiply foot candles by 10.76
  • To convert Lux to foot candles divide Lux by 10.76

5.9. Lambert: It is defined as the brightness of a perfectly diffusing surface, emitting one lumen/ cm2 or having an intensity of 1/π. cd/ cm2. In biological experiments it is used in the form of the milli Lambert (1/1000 of a Lambert). The unit gets the name after Johann Heinrich Lambert.

 

1 foot Lambert (fL) = 1/ π Candela (cd) pe r ft2 = 1.076 mL (or) 3.426 candela / m2 (the corresponding SI unit)

1 milli Lambert (mL) = 0.929 fL

 

5.9.1.   Use

 

It is commonly used for:

  • Measuring luminance of images on a projection screen in the motion picture industry
  • Measuring highlight brightness of visual display systems in flight simulations

5.9.2. Dekalux: is also a measure of luminance and is equal to 1.076 fc

 

6.      Reflection factor or reflectivity (R): The luminance of an object depends on the amount of the incident light which is reflected.

 

All these units are practically based on human vision, as stated earlier and are therefore found to be useful in specifying safe and comfortable levels of illumination in a work area – may it be a house, office, factory, or a commercial centre – a primary field of activity for a space designer.

Professionals in different fields’ are interested in photometric measurements specific to their disciplines. Measurements generally are based on photo detectors, switch type luminaires, light meters and the like. For complex measurements as needed in the lighting industry spherical photometers are used. Tests on lamps and lighting fixtures are done by making use of gonio photometers and/ or rotating type mirror photometers.

 

A light meter is used to measure Illuminance. It can show two parameters, namely, the quantum of light incident on a given area and the quantum of light energy reflected off the surface itself. Use of a foot – candle meter is in practice to determine the luminous intensity of a source. It resembles a type of photographic exposure meter.

 

Summary

 

This session, I believe would have given an insight on the concept of measurement of light, an indispensable element in everybody’s life. Being bombarded with requirements of different kinds on one side and a sweeping array of options on the other, individuals find correct selection quite a difficult task. A little knowledge on these aspects covered today would help anybody to become a prudent consumer.

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