Photometry and Light Measurement

Measuring Light Levels / Photometry

Photometry (measuring light levels) is the science of measurement of visible light in terms of its perceived brightness to human vision.

Light can be measured in several ways. Two of the most frequently used methods used in industry include Lux and foot-candles. Lux (lux) is a unit of illumination of one square meter which is one meter away from a uniform light source. One lux is equal to one lumen per square meter.

An employer has a duty to ensure the health and safety of their employees, which is covered under the health and safety at work act 1974. This act describes a duty to provide lighting that enables work to be carried out safely without putting employees health or eyes at risk.

Regulation 8 of workplace regulations 1992 says that:

• Every workplace should have suitable and sufficient lighting
• There should be natural light so far as is reasonably practicable
• Suitable and sufficient emergency lighting shall be provided where needed

Effects of Bad Lighting

• Too little light may cause fatigue which can result in accidents.
• Approximately 30% of all accidents are directly or indirectly a result of poor, insufficient lighting.

The amount of light needed depends on what type of work is being carried out. (see table below)

Type of Job	Examples	Light Levels (Lux)
Movement of People	Storage area & plant room	150-250
No Perception Of Detail Construction	Loading Bays	300-500
Limited Perception of detail	Factories, Kitchens	500-750
Demanding Work	Inspection, welding, machinery	750-1000
Repetitive Work	Electronics, textile production	1000-1500
Accurate Detail	Technical Offices	1500-3000
Precision Detail	Jewellers & Goldsmiths	3000+

Guide to Typical Outdoor Light Levels

Interior levels are much lower than outdoor natural light.

Some typical light levels are:

Condition	Light Level (Lux)
Very Bright Summer Day	Up to 100,000
Overcast summer day	30,000 – 40,000
Evening televised football match	700 – 16,000
Non-televised football match	200 – 500
Shady room in daylight	250 – 300
Floodlight on a building	60
Night-time in an urban street	10
Night-time in a car park	<1

Light Measurement Meters

The two main instruments for measuring light are photometers and spectrometers,

A photometer is any instrument that can be used to measure illuminance or irradiance. These meters are often applied in industry as ‘photometer’ is a general term for “detecting instruments”. Most photometers work by using either photo resistors or photodiodes. These work by showing a change in electrical properties when exposed to light which is easily detected using a suitable electronic current.

A spectrometer is an optical instrument which is used to measure the properties of light over a specific proportion of its electromagnetic spectrum. Spectrometers use light intensity as the variable measurement and wavelength as the independent variable measurement. Spectrometer is often a term that is applied to instruments that work over a wide range of wavelength from X-rays, gamma rays & far infrared.