Light Measuring Devices

What is called Photometry?

Photometry is the process of measurement of light characteristics using the technique of light measuring devices. It is used in a number of industries to test the intensity of light produced by a source, hoardings, display boards, instrument panels, night-vision devices etc. The following characteristics of light required to measure for specific purposes –

Characteristics of Light

Photometry consists of measuring of four basic characteristics of light. These are –

1. Luminous Flux.
2. Luminous Intensity.
3. Illuminance.
4. Luminance.

1. Luminous Flux –

Luminous flux is defined as the amount of light energy produced by a source of light per unit time in all directions. It has the following unit –

Lumen ( lm ) – It is the SI unit of luminous flux or light energy. It is a unit of light flow from a light source.

2. Luminous Intensity –

Luminous intensity or Intensity of light is the amount of visible light emitted by a source in unit time in a particular direction. Thus Luminous Intensity is the Luminous Flux produced per unit solid angle per unit time by the source. It has the following unit –

Candela ( cd ) – It is the SI unit of luminous intensity. One candela is the luminous intensity of a source that emits mono-chromatic radiation of frequency of \left ( 540 \times {10}^{12} \text {Hz} \right ) in a given direction.

1 \ \text {lumen} = 1 \ \text {Candela} \times \ 1 \ \text {Steradian ( solid angle )} .

Measurement of solid angle ( steradian ) is obtained by the relation –

\omega = \left ( \frac {a}{r^2} \right ) , where ( a ) is the surface area and ( r ) is the radius of imaginary sphere containing that surface.

3. Illuminance –

Illuminance is defined as the amount of luminous flux incident on any surface per unit area of that surface. It has the following unit –

Lux ( lx ) – Lux is the SI unit of measurement of the illuminance i.e. the light intensity falling on a surface.

One lux is equal to one lumen per square meter \left ( Lm / m^2 \right )

Foot-candle – A foot-candle is defined as one lumen per square feet \left ( Lm / ft^2 \right )

4. Luminance –

Luminance is the total light emitted or reflected from a surface in a given direction. It indicates the behavior of the surface and interaction between the incident light and the surface of incident.

Nit – Candela per square meter is called Nit.

It is difficult to have a clear understanding about luminous flux, illuminance and luminance. To understand these, let us consider about a filament light bulb glowing inside a room. Now, consider about the following statements –

1. The amount of light ray produced by the bulb is luminous flux. It is measured in terms of unit lumens. ( measure of light intensity of source ). This indicates the capacity of bulb.
2. This light rays emitted by the bulb fall on walls of room ( a surface ). It is called illuminance of wall surface and is expressed in unit Lux. It is said that, light bulb is illuminating the room.
3. A man sitting in the room can see and feel the extent of this light visually in terms of brightness of walls. It is called luminance of the walls. It is measured in unit candela.

Light Measuring Devices

Different types of light measuring devices are –

1. Photometer.
2. Integrating Sphere.
3. Spectrometer.
4. Light meter or Lux meter.

These devices are explained as under –

1. Photometer –

A photometer is an instrument that measures light intensity. This of two types –

a) Luminance meter –

Luminance meter is also called as Luminous Intensity Meter. It is used to measure the intensity of light produced by a source and quantify the power of that source. These are used for measuring the performance of light source products such as traffic lights, television, head & tail lights used in vehicles etc.

Since luminous intensity measurement is targeted at the source, such measurement is achieved by using an optical lens system. Set up for working of a luminance meter is shown in attached Figure.

b) Illuminance meter –

It measures the intensity of light radiated from a surface far away from light source. Thus it measures the visible light energy falling on any surface.

This device works on the same principle as that of Luminance meter.

2. Integrating Sphere

An integrating sphere is used to collect the emitting electro-magnetic radiations from a source of light.

It measures the Luminous flux to determine the total visible energy emitted by a light source. Hence, it is also called as Luminous Flux meter.

An integrating sphere uses the principle of reflection of light as shown in figure.

A light source, whose capacity has to measure, is kept at the center of integrating sphere. The reflecting inner surface of the sphere, converge all the power emitted by the light source and direct it to the light detector head.

The sphere has to be large enough to easily accommodate the light source being measured. As a general rule, the larger the sphere, the smaller the errors in measuring luminous flux for different light sources.

3. Spectrometer

An spectrometer works on the principle of light refraction and diffraction of light in different wavelengths. Spectrometer take in the emitted light, break it into its spectral components and digitize the signals as a function of wavelength. It is then processed by a computer system into useful information. It is of two types –

a) Optical Spectrometer –

It measures the intensity of light as a function of wavelength or frequency. The different wavelengths of light are separated by refraction in a prism by diffraction. Working of a single beam spectrophotometer is shown in figure below.

b) Optical Emission Spectrometer –

Optical Emission Spectrometer is often called OES or Spark Discharge Spectrometers. It is mainly used in steel foundries to evaluate the composition of alloy steels to determine the percentage of alloying elements with very high accuracy during casting process.

Metal piece is kept between two electrodes. A spark is then applied through a high voltage supply. Due to generation of tremendous heat, the metal surface get vaporizes the metal particles into plasma. The vaporized particles and ions of different materials emit radiations of different wavelengths. These are made to incident on screen of light detectors made of photo-multiplier tubes. The measured data is then processed instantly using a computing system and the results are displayed on monitor.

• Another spectrometer called mass spectrometer is used for identification and segregation of chemical substance.

4. Light Meter or Lux Meter

A light meter or Lux meter is a device for measuring the brightness of a surface. It specifically measure the intensity of light with which the brightness appears to the human eye.

A lux meter works by using the principle of photo-electric cells. The meter capture light through a detector, then converts this light to an electrical current through photo cells. Measuring this current allows the device to calculate the lux value of the light it has captured. This provides the illuminance status of that space.

A general reading of natural illumination at open space is as follows –

1. Summer dry day – 100000 Lux.
2. Winter dry day – 10000 Lux.
3. Full moon sky night – 0.25 Lux.

Required value of illumination at occupational space –

1. Living & Study room – 300 Lux.
2. Hospital operation theatre – 1000 Lux.
3. Lab & Offices – 500 Lux.