When light is released by a liaht source it may be measured and evaluated in various ways. The total light flux could be collected and the quantity indicated in terms of the unit of luminous flux which is the lumen. Luminous flux is the radiant Power of the source evaluated according to the assumed standard sensitivity of the human eye. The area flux density of the lght flux could be measured either incident on a surfae as illuminance (lux or lumens/m2) or leaving a surface as luminous exitance (lumens/m2). The solid angular flux density could be measured as the luminous intensity (candelas or lumens/steradian). The intensity per unit area could be measured as the luminance (cd/m2). Which of these auantities is chosen for measurement would depend upon its purpose. Such measurements can be used to give an indication of the distribution of the emitted light flux in space. However, the effects of the radiation depend not only upon the quantity, intensity and distribution of the radiation but also upon the wavelengths of the radiation present in the light flux. Measurements related to spectral distribution are therefore also required to enable variation of response with wavelength to be taken into account. For visual effects the colour of sources and surfaces are important and so the means of colour specification and measurement can be important for some applications. The measurement of chromaticity coordinates is therefore sometimes reauired. In considering visual effects it is important to appreciate that the relationship between the apparent brightness of a source or surface and the stimulus in terms of luminance is non-linear. Under certain circumstances there is a logarithmic relationship while under others it is a power law relationship. Adaptation and induction effects also modify the practical results and must be considered when interpreting measurements or in setting up experimental procedures.

Photomultipliers are available with effective photocathode diameters from 2.5 mm to 500mm covering a range of scientific and industrial applications. Various photocathode materials are offered for use over the UQV. to near I.R. region of the spectrum. The basic construction and major characteristics are discussed and a brief comparison with other detectors made. Selected applications are given as examples.

The operation of a simple photon counting system is described and the advantages of photon counting and integrated charge measurements are discussed. The selection of a photomultiplier tube for photon counting is considered together with the other components. The relationship between the single electron response, the bias curve and the plateau curve and their use when setting up a photon counting system are discussed. A new photon counting system is described.

Hundreds of luminance telephotometers and microphotometers are in use throughout the world today, but very few of them having scanning microphotometric capability. This paper describes a unique optical system which, without requiring any modification of the photometer's internal optical system, converts an existing photometer into a high resolution scanning microphotometer. The optical system is simply interchanged with the standard objective lens. The system, called the 'MicroScanner Spatial Scanner', will be described in detail. Used with any standard Pritchard Photometer, this scanner provides variable speed scanning of up to 25mms of object space with field coverage as fine as 0.00125 mms. The system features photometric uniformity within ±0.5% over the entire scan distance, freedom from polarization error and a 25mms scan distance.

The errors involved in the short-distance photometry of projectors are evaluated and the same conclusions have been shown to apply to general purpose luminaires. The mathematical analysis from which the equations were derived has been published in Lighting Research and Technology (1981). The illuminance at a short distance from the projector does not follow the inverse square law; the errors depend on the angular subtense of the aperture of the projector relative to the divergence of the beam, and on the distribution of luminance across the aperture of the projector. At any particular distance, the errors are least in directions in which the curvature of the intensity distribution curve is least; the errors may therefore be greatest in the axial direction or in the direction of a shoulder on the curve, and they may change sign where the intensity distribution curve changes from convex to concave. In any particular direction, the error is greater if the outer zones of the projector have higher luminance or give a narrower relative spread; the worst case is a ring-shaped luminaire. If the relative error is less than 10 per cent, it is inversely proportional to the square of the distance of measurement. For general guidance, a nomogram relates the maximum likely percentage error to the beam divergence and to the relative distance of measurement; an empirical reference distance, to be known as the Beam Cross-over Distance, is suggested to replace the traditional 'cross-over distance' of a projector.

The increasing use of retroreflectors for pedestrians in the Nordic countries has led to a need for technical requirements in order to keep the quality of manufactured retroreflectors on a level that is concordant with present-day traffic safety demands. A project group has studied the properties of retroreflectors, including their retroreflection characteristics and their degradation with use which results in finite life time. The findings of the group has resulted in a proposal for a common Nordic set of test specifications for pedestrian retroreflectors.

A brief review is made of the major factors that are likely to cause eye discomfort from clerical work in traditional office environments. A procedure is proposed whereby cases of complaint may be systematically investigated. Methods are suggested for making a fairly quick evaluation of the task characteristics and the lighting conditions and it is suggested that measurements of the thermal characteristics of the environment also be carried out. In order to complete the investigation, it is necessary to obtain a person's subjective responses to the environment, using a suitably prepared questionairre, and for an eye examination to be performed by an ophthalmic optician.

The CIE has divided the ultra violet spectrum into three regions: the UVA 315-400nm, the UVB 280-315nm and the UVC 100-280nm. Radiation in the UVC region does not occur in the solar spectrum, UVB is the erythermal or "sun burn" producing region and the UVA is long wave or black light region. Electric discharge lamps in the form of fluorescent and high intensity discharge lamps are very widely used in commercial and industrial lighting because of their high efficiency. Due to their mode of action, they radiate a measurable amount of radiation in the UVA region and in some cases in the UVB region. Tungsten halogen lamps also emit some ultra violet radiation. This paper discusses in elementary terms the construction and mode of action of various lamps and the measurement of the ultra violet radiation. It also examines the ultra violet radiation in terms of the National Radiological Protection Board Specification for general lighting application. The special applications of UVA and UVB sun lamps is also discussed.

The ultraviolet radiation (UVR) radiometry of solar simulated radiation in a long-term photocarcinogenesis project is described. The methods used were (a) a phototherapy radiometer, (b) an electronic integrating dosimeter, (c) indirect spectroradiometry,and (d) polysulphone and naladixic film badge dosimeters for UV-B (280-315 nm) and UV-A (315-400 nm) radiation, respectively. The merits of the various methods are discussed. The importance of reliable and practical UVR radiometry is emphasised.

HSE Guidance Notes are being prepared to give recommendations to the designers, manufacturers, operators and users of commercial sun tanning equipment on the various health and safety aspects associated with the safe construction, siting and use of such equipment. Medically prescribed ultraviolet treatments are excluded from the guidance.

A number of radiometric and photometric standards have been established by the various national standards laboratories. These basic standards can be used to calibrate a wide range of optical radiation measurement instrumentation. However, in many instances these basic standards are not suitable for calibrating the instrumentation used in many research investigations. In order to fulfill this need, a number of special purpose calibration standards have been set up. This paper briefly describes some of the more widely used basic standards and, in more detail, describes some of the special purpose calibration sources which are available for calibrating various photometers, radiometers and spectro-radiometers.

Photometric standard lamps having a low luminous intensity and a colour temperature greater than about 2700 K have undesirably short lives. A new standard lamp which is being developed at NPL in collaboration with the GEC Hirst Research Centre should overcome this problem. It consists of a vertical ribbon filament lamp with the area of the lamp wall in front of the filament cut off from the rest of the lamp by an internal boxed screen. At the centre of this screen is a small circular aperture which limits the effective area of the source. The luminous intensity of the lamp is equal to the area of the aperture multiplied by the luminance of the filament. Since a ribbon filament lamp has a reasonable life when run at the colour temperature of Standard Illuminant "A" and since the aperture can be made small, an intensity of the order of a few candelas is practicable yet with a good life. In addition the light is virtually unpolarized. The possibility is also envisaged of using alternative and quicker methods of accurately calibrating such lamps and of making lamps with vertical slit apertures for use with spectroradiometers.

The difficulties encountered in simulating accurately CIE Standard Illuminant D6, representing average daylight, particularly in the ultraviolet region, are considered. Accurate simulation is needed for colour measurements using colorimeters and for spectro-photometric measurements on fluorescent samples using polychromatic illumination. The ultraviolet content of a simulator can only affect the colour of a fluorescent material and the two main classes of such materials are considered in terms of how critical the ultra-violet content is. Samples were polychromatically illuminated with approximate simulations of CIE Standard Illuminants D65 and C and spectral total radiance factors measured. These results were corrected to represent the spectral total radiance factors that would be produced for ideal illuminants, by using additional quantities (true spectral reflectance and the emission and excitation spectra) determined from measurements on the samples by the NPL Spectrofluorimeter. The colour specifications of the samples were computed using the corrected spectral total radiance factors and indicate that Illuminant C was an adequate substitute for D65, for a sample representative of fluorescent signal and safety colours. An illuminant with an ultraviolet content similar to interior daylight, daylight attenuated by average window glass, was sufficient to produce an effect similar to that for D65 for a fluorescent white sample. Such a reduction in requirements for the shortest wavelengths present in a daylight simulator would allow the reproduction of the visible region to be imoroved and cheap, reliable simulators to be produced, without reducing the accuracy with which the colour specification of a fluorescent material can be measured.

Traditionally, determining the spectroradiometric output of a light source has been a long, tedious, rather time consuming process. Many hours could be spent in calibrating the spectroradiometer for spectral response, collecting data on the source under investigation and in properly reducing the data. Due to rapid advances in electronic technology, it is now possible to obtain an automated spectroradiometer system at a modest cost which can be interfaced to a relatively inexpensive programable desk top calculator or microcomputer to provide a system which can automatically conduct a spectral scan and provide a real-time printout or display of spectral output under program control. This paper will cover the basic parameters to consider when selecting a spectroradiometer and also the features a spectroradiometer should have in order to be completely automated and capable of operating under programed control. In addition, requirements for measuring a wide range of light sources ranging from continuous or steady state sources from sunlight to starlight and also pulsed sources such as Xenon flash lamps and pulsed LED's will be briefly described. Finally, some of the present commercially available automated spectroradiometer systems will be described.

There has been widespread international concern expressed over the potential exposure of visual display unit (VDU) operators from radiations emitted by the VDUs. In order to investigate this potential exposure situation and to determine the extent of any such radiant energy exposure, a survey was organised to measure the radiation emitted in all regions of the electromagnetic spectrum from all VDUs currently available in the UK. With the exception of some special tests, all the surveys were carried out at manufacturers/suppliers' premises. In all, some 60 firms were visited and over 200 different models of VDU were surveyed using a common instrument/measurement protocol. All measurements were made with the detector either in contact with, or in close proximity to, the screen and all accessible sides of the casing. The brilliance and contrast controls of the VDU were adjusted to the maximum setting consistent with an acceptable display. The field survey measurements were made by National Radiological Protection Board staff acting on behalf of the Health and Safety Executive.

The introduction of projection screens with high gains has had an important effect on the development of projection television. These screens have very directional reflectance characteristics which have been measured by a specially built goniophotometer. The measurements from the instrument give a direct indication of the field of view that can be obtained from a screen for a given fall in apparent luminance. The measured results can also be used to calculate some of the less obvious photometric and colorimetric consequences of using and misusing these screens.

Silicon photodetectors arelwidely used in the electro-optics industry for a variety of light detection and measurement applications. They offer the advantages of low cost, relatively high speed detection of light with usefully high quantum efficiency in the 0.3 to 1.1 micron wavelength region. In this review, we discuss the various trade-offs that are available to the silicon photodiode manufacturer in optimising the performance of a device for a particular application. This should enable the optical system designer to obtain a clearer understanding of the device parameters relevant to his particular needs, thereby obtaining a better match between device and system engineering constraints.

A recently developed deep-sea photometer is described. The photometer uses a silicon photodiode as a light sensor and is interfaced with the Institute of Oceanographic Sciences' acoustically telemetering net monitor system. Using the photometer in conjunction with the I.O.S. midwater trawls it is possible to simultaneously measure irradiance at the depth of the net and collect biological samples. The relationship between light and the distribution of mesopelagic animal communities can thereby be studied directly. The photometer has measured light to a depth of 700m in the northeast Atlantic. It records at least seven decades of irradiance and its response is independent of temperature between -3 and +30°C. Some observations on the subsurface light regime in the northeast Atlantic and Southern Oceans are presented and a biological sampling programme using the photometer is described. Preliminary analysis suggests that the association of oarticular species with specific light levels is not especially close.

Recent studies have determined the efficiency of crop production by relating the rate of increase of dry matter in healthy growing crops to the interception of sunlight. In addition to knowledge of the incident light, such studies require measurement of light transmission in the crop, or timely information about leaf area for light interception to be estimated. Transmission measurements are necessarily confined to small areas while traditional methods of determining leaf area are laborious and often require destructive sampling of part of the crop. Remote sensing techniques offer a cheap, non-destructive system for sampling large areas. An airborne sensor is used to detect solar radiation reflected from crops in two spectral bands: the near infra-red band (780 - 940 nm) is strongly reflected by leaves due to the porous structure of the mesophyll; the red band (600 - 660 nm) is strongly absorbed by chlorophyll in the leaves. The ratio of red/infra-red reflected fluxes decreases with the percentage cover of healthy green leaf and is largely independent of the effects of varying solar irradiance. Measurements made over sugar beet showed that during the main period of growth, spectral ratios were linearly related to leaf cover and light interception. There was some evidence of hysteresis later in the season when the spectral ratios tended to increase in spite of constant leaf cover, and this may indicate senescence of the leaves and loss of chlorophyll. These relationships are consistent for a wide variety of crops and allow the light interception by the crop to be estimated by a single spectral measurement from above. This information may be used to predict future rates of growth and ultimately, crop yields.