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Description
Precision diffuse optical emitters (PDOEs) based on integrating spheres are used for radiometric calibration of digital cameras, optical sensors, and optoelectronic systems, in particular for measuring response linearity and spatial nonuniformity and for determining correction parameters of focal plane arrays. In modern PDOEs, the integrating sphere mixes radiation spatially, while high-power light-emitting diodes (LEDs) provide compactness, high luminous efficacy, and electronic flux control. Long-term stability of the emitter is critical to measurement reproducibility.
LEDs differ in luminous flux, spectral power distribution, forward voltage, and flux-current dependence. These characteristics, which can be nonlinear, vary with supply conditions, junction temperature, and operating time. Changes in the contributions of individual LEDs alter the luminance of the exit aperture and degrade the uniformity of its luminance distribution.
To ensure long-term stable operation, a modular automatic control system with individual LED current control, optical feedback, and temperature compensation is being developed. The PDOE contains three high-power Cree XM-L2 LEDs with active cooling inside an integrating sphere (internal diameter 266 mm, exit aperture diameter 50 mm). Illuminance was measured with a TENMARS TM-208A meter. After preliminary channel balancing, the illuminance with all LEDs on, 40 mm from the exit aperture, was 1773, 7530, and 7820 lx in the three investigated modes (highest-to-lowest ratio 4.41:1). In the highest-illuminance mode, currents of 1840, 2100, and 1790 mA produced 2643, 2639, and 2640 lx for the individual LEDs. These results support independent control and individual calibration of each LED channel.
The system comprises a control computer, an independent constant-current driver for each LED, and digital modules communicating with the computer via a serial interface. Optical feedback will use a photodiode channel with operational amplifiers, analog-to-digital conversion, and spectrum-dependent calibration; a multichannel spectral sensor will monitor visible spectral bands, and thermocouples will measure circuit board temperatures. Proportional-integral illuminance control will be combined with temperature calibration models and a slower spectral correction loop. LED currents are derived from a current-temperature-luminous flux characteristic matrix obtained by individual LED calibration within electrical and thermal limits.
The software lets users set the exit-aperture illuminance or luminance, a reference spectral response, current and temperature limits, per-LED characteristic matrices, and measurement parameters. It displays illuminance, responses in eight spectral bands, temperatures, current setpoints and deviations, time plots, and alarms in text and graphical form, keeps operating conditions, including supply voltage and LED temperature, within limits, and logs all measurements and applied corrections.
The proposed system is expected to provide long-term stable PDOE operation with an adjustable exit-aperture brightness held constant during radiometric calibrations. This would enable serial production in Ukraine of PDOEs with precision comparable to leading international manufacturers at a considerably lower price.
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