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Keo Alcor Low Brightness Source

The Keo Scientific Low Brightness Source (LBS) provides a lightweight, portable means of calibrating low-light-level devices such as airglow and auroral imagers and photometers as well as other night-vision devices. Its small size makes it particularly useful for instruments that must be calibrated in situ, such as may be found mounted on aircraft, manned spacecraft, or used in field environments.

The Keo LBS consists of a 50W precision quartz tungsten-halogen bulb mounted in a thermally controlled box. The unit contains a sophisticated system of holographic attenuators and diffusing screens. Overall dimensions are 7" x 7.5" x 16.5" (15 lbs), with a 100 mm diameter output screen.

With the opal output diffusing screen in place, the unit serves as a standard of spectral radiance and as a luminance standard. With the screen removed, the unit can be used as a standard of spectral irradiance and as an illuminance standard.

A separate control unit contains a highly regulated power supply, precision current sensing resistor, digital panel meter, and optional elapsed-time meter. Two 10-turn adjustment potentiometers, for Coarse and Fine adjustment, respectively, are provided for trimming lamp current before use.

Conventional incandescent lamps decline as much as 30% in light output during their lifetime, primarily due to bulb darkening from evaporated tungsten deposits. Consequently, a tungsten-halogen was chosen for this LBS. The lamp has a tungsten filament mounted in a quartz envelope containing a small amount of halogen gas. The lamp operates at a high temperature at which the halogen gas combines chemically at the bulb wall with the evaporated tungsten. The resulting tungsten-halogen gas then migrates to the hot filament where it decomposes and deposits tungsten back onto the filament. This cycle continually cleans the bulb so that the lamp maintains over 95% of initial light output throughout its rated lifetime.

The spectral distribution from the filament through the quartz envelope in the UV and Visible is essentially the same as a Black Body radiator at the lamp's apparent color temperature of 3000K.

[ Larger photo | Brochure for the Remote Controlled version (Alcor-RC) ]

Lamp Features
- Precision quartz halogen lamp
- Pre-focussed filament
- 3000 degrees Kelvin color temperature, for enhanced blue output
- 2000 hours rated lifetime
- Optional elapsed-time counter (Lamp Hours display)

Optical Features
- Light attenuation by means of field stop discs (via manual thumbwheel or remote servo motor control)
- Micro-EDM machined field stops
- Internally sandblasted and fully baffled
- Total Attenuator Dynamic Range: 10^6
- Proprietary combination of holographic diffusers, ground glass, and flashed opal on optical crown
- Highly uniform Lambertian output
- Available with or without absolute calibration (as some prefer to conduct their own calibration)

Electrical
- Power-on sequence: lamp ramps up to full voltage over about 5 seconds, in order to maximize lamp life
- Current drift < 1 mA over 3 hours, after initial 5 minutes warm-up period
- 5 min. warm-up period required to allow bulb, precision resistor, flux capacitor and PSU to fully stabilize
- Power supply operated in remote voltage sense mode (i.e., using Sense Lines)
- Bulb overvoltage protection circuitry
- Precision engineered (Swiss-made) LEMO® connectors
- Mains operating voltage: 110-220 VAC (optionally 12 VDC)

Mechanical
- CNC machined, black anodized aluminum
- Sealed, light-tight construction
- Knurled aperture wheels, for easy operation
- Bulb is field replaceable
- Accepts both 100 mm and 4 inch diameter output screens, by means of reversible retaining ring
- Stable rubber-feet desk stand, balanced around center of gravity
- Tripod threads provided for maximum flexibility: ½-20, 3/8-16, and M6