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It is defined as the RATIO of horizontal distance between ADJACENT lamps and HEIGHT of their MOUNTINGS.

Space-height ratio = [Horizontal distance between two adjacent lamps] / [Mounting height of lamps above working plane]

It is defined as the ratio of horizontal distance between adjacent lamps and height of their mountings.

Space-height ratio = [Horizontal distance between two adjacent lamps] / [Mounting height of lamps above working plane]

• In most lamps or sources of light the LUMINOUS INTENSITY is not the same in all directions. 
• If the luminous intensity, IE, the CANDLE POWER is measured in a horizontal plane about a vertical axis and a curve is plotter between candle power and the angular position, a curve obtained is called as horizontal polar curve.
• The luminous intensity in all the directions can be represented by polar curves. 
• If the luminous intensity in a vertical plane is plotted against position, a curve known as vertical polar curve.

1. Bunsen head or Grease spot PHOTOMETER
2. LUMMER-brodhun photometer head

There are two TYPES of lummer brodhun heads:

1. EQUALITY oc Brightness type photometer head
2. Contrast type photometer head

There are two types of lummer brodhun heads:

It is DEFINED as the RATIO of total LUMENS reaching the working plane to total lumens given out by the LAMP

utilisation factor = [Total lumens reaching the working plane] / [Total lumens given out by the lamp]

It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp

utilisation factor = [Total lumens reaching the working plane] / [Total lumens given out by the lamp]

As PER the principle of operation the light SOURCES MAY be grouped as follows:

1. Arc lamps
2. High temperature Lamps
3. Gaseous DISCHARGE Lamps
4. FLUORESCENT type Lamps

As per the principle of operation the light sources may be grouped as follows:

The mean of candle power in all DIRECTIONS in the horizontal PLANE containing the source of LIGHT is TERMED as Mean Horizontal Candle Power.

The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power.

It is defined as the number of LUMENS GIVEN out by the SOURCE in a unit solid angle in a given direction. 

It is DENOTED by CP

Cp = lumens /ω

It is defined as the number of lumens given out by the source in a unit solid angle in a given direction. 

It is denoted by CP

Cp = lumens /ω

Law of Inverse squares:

Illumination at a point is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.

• If a source of light which EMITS light equally in all DIRECTIONS be placed at the centre of a hollow sphere, the light will fall UNIFORMLY on the inner SURFACE of the sphere. 
• If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius.

Lambert’s COSINE law:

The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the normal to the surface at that point.

Law of Inverse squares:

Illumination at a point is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.

Lambert’s cosine law:

The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the normal to the surface at that point.

It is defined as the luminous per UNIT PROJECTED area of either a SURFACE source of light or a reflecting surface and is DENOTED by L.

It is defined as the luminous per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.

It is defined as the total QUANTITY of LIGHT energy emitted per second from a luminous BODY. 

• It is represented by symbol F and is measured in lumens. 
• The conception of luminous flux helps us to specify the output and EFFICIENCY of a given light source.

It is defined as the total quantity of light energy emitted per second from a luminous body.