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Frequently Asked Questions on Performance of LaserMethane® Detector


1. Radiating angle of the laser beam
2. Co-axial configuration of the sensing light and the guide light
3. Meaning of the offset measurement
4. Calibration




1. Radiating angle of the laser beam

Is it better to have wider beam?

LaserMethane® Detector(LMD) identifies gas leakages by detecting reflected laser light, which originally emitted from LMD. You may think it is always better to have wider beam angles because the spreading light could increase scanning efficiency.


Fig.1 Is it better to have a wider beam in order to increase scanning efficiency?

The detector receives only reflected light parllel to the lens axis.

The laser light emitted from the light source will be randomly reflected at target surfaces; therefore, the lens facing to the front receives light from all the points of the target surface. The question is how much light going to the lens can be utilized for the leak detection when considering efficiency.


Fig. 2 shows trajectories of light going through the lens. Note that no light will focus on the detector except light normal to the lens surface. This observation is based on physics you learn at elementary schools, but this leads to an important fact that laser light impinges on the projected area of the aperture of the lens will only contribute to the leak detection. No light from the perimeter area can contribute the detection at all because the light from the perimeter will not go into the lens normally.



Fig.3 Trajectory of light. Light from the center region will contribute to the measurement. The detector will not detect light from the outside


Widening laser beam does not only contribute to the leak detection, but also lose precious light power on the effective area in the middle. If leak detection at a distant point is required, narrowing the beam width is rather beneficial.



2. Co-axial configuration of the sensing light and the guide light

Aiming targets without parallax

The red marker light is necessary to locate the target because our eye is not sensitive to the sensing infrared light. Co-axial configuration of the red marker and the sensing light, which is employed in LMD design, is important when the distance to the target is relatively short. Parallax, well known phenomena in range finder cameras, will introduce errors between the marker and the sensing points.


Fig 4. Configurations of the sensing light and the marker light



3. Meaning of the offset measurement

Subtracting ambient methane concentration in air

Very small concentrations of methane gas at an average of a few ppm does exist in ambient air, and it always fluctuates spatially and temporary. On the other hand, the quantity measured by LMD is not the methane concentration but the intensity-column density, which is a product of the gas concentration and its existing distance. When a long distance between the target and LMD is expected, the effect from the ambient methane may not be negligible. The offset function is to mask the effect from the ambient methane. Note that the amount of offset depends on location and target distance so that it cannot be a constant.



4. Calibration

Precise frequency control of laser oscillation eliminates awkward calibration procedure

Calibration circuit, detector, laser beam, methane cell, laser diode, frequency control circuit, optics, laser light


Fig.5 Block diagram of frequency calibration

The frequency of laser oscillation has to be precisely controlled to the methane absorption. Adjusting electric current and ambient temperature of a laser diode performs this adjustment. Molding the laser diode unit and a glass cell containing methane gas, a proprietary technology of Anritsu, enables continuous frequency adjustment even during the normal operation, and this completely eliminates occasional calibration procedures.