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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®standards | OPTICAL SAFETYIn measuring irradiance, the measurementoptic, typically a diffuser or an integratingsphere, should have a cosine angularresponse to correctly account for off-axiscontributions. At a given angle from the surfacenormal, the projected area on the surfaceis increased by the cosine of the saidangle, resulting in reduced irradiance.Knowledge of source irradiance doesnot however give any information aboutthe quantity of light coupled by the eye andimaged on the retina, for which a measurementof radiance is required.Measurement of radianceRadiance permits the evaluation of hazardsto the retina. Radiance is defined as the ratioof radiant power (dΦ) emitted by area dAinto solid angle dΩ at angle θ to the sourcenormal, to the product of solid angle dΩ andthe projected area dA∙cos θ (Fig. 2). The symbolis L and the units are W/m²sr.In viewing a source, the eye collectsSourceSourceFSolid angle, ΩγObject distance, S1Field stopγrAperture stop, DFIG. 3. Measurement of radiance: imaging technique.FIG. 4. Measurement of radiance: indirect technique.Image distance, S2γCosine correctedinput opticSkin & EyeRetinalight within a given solidangle set by the diameterof the pupil, and projectsan image of thesource onto the retina. Asthe pupil dilates (or contracts)according to thelevel of visual stimulus, orluminance, of the source,the retinal irradiance ofthe image increases (ordecreases).The law of conservationof radiance statesthat radiance cannot beincreased by passive opticalsystems such as the lens of the eye. Theretinal irradiance is therefore determinedfrom the source radiance and the solid anglesubtended by the pupil (2-7-mm diameter) atthe retina (17-mm distant) in the reverse ofthe determination of radiance from irradiance,given below.Imaged, Aperture(field stop)HazardWavelengthrange (nm)QuantitymeasuredActinic UV 200-400† IrradianceNear UV 315-400 IrradianceIR Radiation Eye 780-3000 IrradianceThermal Skin 380-3000 IrradianceBlue Light Small Source 300-700† IrradianceBlue Light 300-700† RadianceRetinal Thermal 380-1400† RadianceRetinal Thermal Weak Visual 780-1400† RadianceTABLE 1. Different hazards require the measurementof either irradiance or radiance. († Weighting functionrequired.)Radiance may be measured by two manners,either using an imaging technique orindirectly through an irradiance measurement.In both cases, the measurement is performedin a specific field of view (FOV) orsolid angle of acceptance (often described bya planar angle, θ) that defines the area of thesource measured.The imaging technique (Fig. 3) replicatesthe imaging of the eye. A telescope imagesthe source under test onto a plane at whichmay be placed apertures of varying diameterto select the required FOV of measurement.Alternatively, a measurement of irradiancewith a cosine-corrected input optic maybe performed (Fig. 4). An aperture is placeddirectly at the source to define the measurementFOV. The radiance is computed fromthe ratio of irradiance to the solid angle ofthe FOV in steradians.Physiological radianceFor momentary viewing , the retinal imageof a source subtends the same angle as doesthe source. The smallest image formed onthe retina, according to IEC62471, has anangular extent of 1.7 mrad, given the imperfectimaging performance of the eye.With increasing exposure time, due to eyemovement (saccades) and task-determinedmovement, the retinal image is smeared overa larger area of the retina, resulting in a correspondingreduction in retinal irradiance.A time-dependent function is defined to representthe spread of the retinal image in therange from 1.7 to 100 mrad. This covers therange from 0.25s (aversion response time) to10,000s exposure.66 NOVEMBER/DECEMBER 2011 LEDsmagazine.comPrevious Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®