Basics for measuring optical radiation

What is ´Optical Radiation´?

Optical radiation covers the wave length range from 100nm to 1mm of the electromagnetic radiation spectrum.
It must be considered that, with regard to the range limits, they do not preset a sharp separation, which is compulsory for all applications.
The detection of optical radiation can, for example, be measured by means of radiometric, photometric, photobiological or plant-physiological measurable variables.


Definition of Photometric and Radiometric Measurable Variables

Limited to the range of the optical spectrum (light) that is visible to the human eye. Photometric measurable variables include: Light flux, illuminance, luminance and luminous intensity. The main characteristics of photometry is the evaluation of the brightness perception by the spectral luminosity function of the eye for photopic vision or, in rare cases, for scotopic vision (DIN 5031). Radiation detectors for photometric measuring tasks must, therefore, provide one of these spectral response characteristics.

Light Flux
The luminous power of a light source (lamp, LED etc.). As lamps do not generally emit a completely parallel luminous beam, the light flux measurement is performed by using measurement geometries, which detect the light flux independent from its geometric distribution. In most cases Ulbricht globe photometers or goniometers will be used.

Luminous Intensity
The part of a light flux, which radiates in one specific direction. The luminous intensity is an important variable for calculating the efficiency and quality of lighting equipment. The measurement is performed by detectors with a defined field of view and placed at distances that allow to consider the light source as a point light source.

The brightness sensation provided by an illuminated or luminous surface to the eye. In many cases the luminance data will provide significantly better information regarding the quality of a light than the illuminance. For measuring the luminance, measuring heads with a defined measuring field angle are used.

The light flux of one or several light sources striking a certain surface horizontally or vertically. In case of a non-parallel incidence (which is the typical case in practical photometry) a cosine diffusor must be used as measurement geometries.

Metrological evaluation of optical radiation using the radiometric variables „Radiation Capacity“, „Radiant Intensity“, „Radiancy“ and „Intensity of Irradiation“. The main characteristic of radiometry is the wavelength-independent examination of the intensity of radiation. This is the significant difference between radiometry and actively weighted measurable variables, such as variables used in photometry, photobiology, plant physiology etc.

Radiation Capacity
The overall power provided by radiation.

Radiant Intensity
The quotient from the radiation capacity emitted by the light source into a certain direction and the solid angle being covered. The radiant intensity is used for the measurement of the geometric distribution of the radiation capacity.

The quotient from the radiation capacity passing through (striking) a plane in a certain direction and the product of the passed solid angle and the projection of the plane to a plane surface, which is perpendicular to the examined direction. The radiancy is used for the evaluation of aperture radiators. Steradian or telescopic adapters can be used as measurement geometries.

Intensity of Irradiation
The quotient of the radiation capacity striking a plane and the illuminated plane. For measuring the intensity of irradiation the spacial examination of the incident radiation is very important; therefore, a cosine-corrected field view function has been preset.

Comparison of Photometric and Radiometric Variables

Every photometric variable corresponds to a radiometric variable and involves the same interrelationships between them. The variables can be distinguished by their index v (visual) and index e (energetic).


Spectral Valuation Function

The relative spectral sensitivity of the human eye is specified with different functions for the light-adapted eye (photopic vision) or for the dark-adapted eye (scotopic vision). Due to the individual differences this data can only be considered for average values but is sufficient for most technical purposes. The detailed data of the spectral sensitivity curve are given in table format in the DIN 5031 standard.
The two different spectral action functions result from the different „sensor types“ of the eye.
The relative luminous efficiency for photopic vision (rods, > 10cd/m2) is described with the function V(l), which is the function used in most cases. The spectral luminous efficiency for the scotopic vision (cones, < 0.001cd/m2) is described with the function V’(l) and can, with regard to the practical use, only be rarely found.

Determination of Photometric Characteristic Factors

The metrological evaluation of the properties of materials regarding their reflection, transmission and absorption, as well as the stray light of objectives, is based on internationally accepted recommendations. These mainly include the CIE 130-1998 „Practical methods for the measurements of reflectance and transmittance“, DIN 5036 Part 3 „Radiometric and photometric characteristics of materials“, DIN 67507 „Light transmission factor of glazing“, DIN 58186 „Stray light determination of optically image-forming systems“.


Why Measure Optical Radiation?

A large part of the human sense impression is of an optical nature. Light is the only visible part of the electromagnetic spectrum. The human eye perceives different wave lengths of the light as colours. The spectral response of the eye, with regard to different colours, depends on the wave length. Furthermore, the human system is also influenced by ultraviolet radiation in a short-wave range and the infrared radiation in a long-wave range of the electromagnetic spectrum.

People are used to daylight illumination. This can be approximately 5000 lux on a dull winter day, while on a sunny summer day approximately 100000 lux are reached. In contrast, only between 100 and 1000 lux are reached with artificial illumination. However, sufficient light is an essential factor for the well-being of people. Symptoms of tiredness, caused by insufficient light, do not generally occur at the eye but affect the whole body.
The standard DIN 5035/2, therefore, contains illumination standard values for health protection at work places.
These are legally bound in the guideline ASR 7/3 and it is imperative that this is observed.
The following nominal illuminations are valid for inside:

Offices: office rooms 300 Lux
work places for writing and drawing 750 Lux
Factories: visual works within the production process 1000 Lux
Hotels: recreation rooms, reception, counter (cash) 200 Lux
Shops: front side of show windows 1500 – 2500 Lux
Hospitals: patients´ rooms, 100 – 150 Lux
emergencies 500 Lux
Schools: lecture rooms, gymnasiums 300 Lux

Global Radiation:
The global radiation is a measuring variable that is especially important for environmental research. It represents the entire diffuse and direct sun radiation that strikes the surface of the earth. The spectral range covers wavelengths from the short-wave range, at 300nm (UV-B), to the long-wave range, at 5000nm (IR).

UVA Radiation:
The long-wave UV radiation (more than 313nm) reaches the surface of the earth almost unfiltered and tans the human skin and strengthens the immune system. In solariums the biological effect of the UVA spectrum is used, combined with other spectral ranges, to trigger the direct pigmentation (melanin colouring). Damages to the connective tissue and premature skin ageing are promoted by too much radiation.

UVB Radiation:
The short-wave UV range (less than 313nm) can cause irreversible damages. All spectral characteristic functions that can have unfavourable effects on the human skin are summarised in the CIE recommendation. This recommendation is described in DIN 5050 and regarded as a guideline. A popular measure for the ‚sunburn sensitivity‘ is, for example, the UV index ‚UVI‘ provided by the German Weather Service. The measuring results provide, directly or in comparison with other spectral ranges, information that is of medical or biological relevance.