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S1881 Datasheet(PDF) 10 Page - List of Unclassifed Manufacturers |
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S1881 Datasheet(HTML) 10 Page - List of Unclassifed Manufacturers |
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10 / 16 page  7 Figure 4-2 shows the photocurrent output example from electrodes of a one-dimensional PSD with a resistance length of 3 mm (S4583-04, etc.), measured when a light beam is scanned over the active surface. The position de- tection error estimated from the obtained data is also shown in the lower graph. The light beam position can be detected over the entire ac- tive area of PSD. However, if part of the light beam strikes outside the active area, a positional shift in the center-of- gravity occurs between the entire light beam and the light spot falling within the active area, making the position measurement unreliable. It is therefore necessary to select a PSD whose active area matches the incident spot light. Figure 4-1 Cross section of PSD KPSDB0005EA Figure 4-2 Photocurrent output example of one- dimensional PSD (S4583-04, etc.) Position detection error example of one- dimensional PSD (S4583-04, etc) Specific area for position detection error KPSDC0071EA Position detection capability is the most important charac- teristic of a PSD. The position of a spot light incident on the PSD surface can be measured by making calculations based on the photocurrent extracted from each electrode. The position obtained here with the PSD is the center-of- gravity of the spot light, and is independent of the spot light size, shape and intensity. However, the calculated position usually varies slightly in each PSD from the actual position of the incident light. This difference is referred to as the “position detection error” and is explained below. If a light beam strikes the electrical center of a PSD, the signal currents extracted from the output electrodes are equal. When this electrical center is viewed as the origin, the position detection error is defined as the difference be- tween the position at which the light is actually incident on the PSD and the position calculated from the PSD outputs. Characteristic and use 4. Position detection error KPSDC0073EA Figure 4-3 Center-of-gravity of incident spot light In Figure 4-1 above, if the actual position of incident light is Xi and the position calculated by the photocurrents (IX1 and IX2) from electrodes X1 and X2 is Xm, then the difference in distance between Xi and Xm is defined as the position detection error as calculated below. Position detection error E = Xi - Xm [µm] ........ (4-1) Xi : Actual position of incident light (µm) Xm: Calculated position of incident light (µm) The position detection error is measured under the follow- ing conditions. · Light source : λ=890 nm · Spot light size : φ200 µm · Total photocurrent: 10 µA · Reverse voltage : Specified value (listed in data sheets) Xm = . IX2 - IX1 IX1 + IX2 LX 2 ........ (4-2) POSITION ON PSD (mm) +1.5 0 -1.5 0 0.5 1.0 IX2 IX1 +50 0 -50 -1.5 -1.0 -0.5 +0.5 +1.0 0 +1.5 POSITION ON PSD (mm) OUTPUT ELECTRODE X1 OUTPUT ELECTRODE X2 CENTER-OF-GRAVITY OF SPOT LIGHT FALLING WITHIN ACTIVE AREA CENTER-OF-GRAVITY OF ENTIRE SPOT LIGHT SPOT LIGHT ACTIVE AREA COMMON ELECTRODE RESISTANCE LENGTH LX SPOT LIGHT Xm Xi X1 X2 CALCULATED POSITION Xm ACTUAL POSITION Xi ELECTRICAL CENTER B N LAYER I LAYER P-TYPE RESISTIVE LAYER |
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