An optical method for direct determination of the radiometric aperture area at high accuracy

Abstract

An optical method for calibration of the aperture area is described and studied both theoretically and experimentally. A spatially uniform, known irradiance is formed over the aperture by overlapping identical, parallel laser beams centred at constant spacing in an orthogonal lattice. The ratio of the throughput power and irradiance gives the area of the aperture. The method has several advantages compared with previous methods: it measures the area of the aperture directly, the shape of the aperture is not limited to a circle, it is relatively inexpensive to establish, it does not damage the edges of the aperture and the calibration set-up is similar to that for the actual use of the aperture. It is estimated that the relative standard uncertainty is 1.6 × 10 −4 in calibration of a circular 3 mm diameter aperture. The results that the present method gave for one aperture have been compared with the result of a mechanical calibration at the National Physical Laboratory (UK). The relative difference between the results was 2.4 × 10 −4 , with a combined standard uncertainty of 2.5 × 10 −4 .

Optical aperture area determination for accurate illuminance and luminous efficacy measurements of LED lamps

“…The laser scanning method is described in detail in [18][19][20]. Therefore, only a brief introduction is given here.…”

“…It was estimated in [18] that relative throughput of 0.9999 requires the ratio of the aperture diameter D a and the e -2 diameter of the beam D b to satisfy the condition…”

“…Several methods for determining the area of an aperture have been developed, which utilize either mechanical contact [16,17] or optical techniques [18][19][20][21][22][23][24][25]. In addition, a spatially uniform irradiance source can be used to compare the area of two apertures [16,26,27].…”

“…While most methods measure the diameter of a round aperture, some measure the area directly and can be used to measure irregular apertures. Lassila et al developed such an optical method for direct determination of aperture area [18][19][20], in which a two-dimensional superposition of equally spaced Gaussian laser beams is used to form a uniform irradiance distribution. In practice, this is accomplished by having an intensity-stabilized laser beam and scanning the aperture in front of the laser beam.…”

“…The method has been validated by comparing the measured area of apertures of different sizes to the area determined using mechanical contact methods [18][19][20].…”