JPS6113207B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is for accurately detecting the position and time of a light spot with respect to changes in light output when scanning a light spot.

BACKGROUND ART Recently, high-speed precision scanning of laser beams using optical deflectors, lenses, etc., has become popular in lasers and printers. At this time, in order to accurately read the position and time of the light spot, an optical scanning position detector is usually placed in the optical scanning optical path. For example, when considering a laser printer optical system for scanning a photosensitive drum 6 as shown in FIG.
The aim is to accurately detect the position of the scanned light spot. Based on the signal from the detector 7, the writing start position is controlled, the modulator 2 is operated, and the laser light source 1 is
By ON-OFF modulating the light beam from the
It is an attempt to draw a figure. In FIG. 1, 3 is a reflecting mirror and 5 is a lens. This optical scanning position detector can prevent deterioration in image quality due to variations in writing positions caused by manufacturing accuracy of the polygon mirror, uneven rotation of the motor, and the like. A photodiode or phototransistor is usually used as the detector. The photodetected signal passes through a comparator and becomes a digital signal. Therefore, when performing binarization, it is common to provide a threshold value in the comparator. For example, as shown in FIG. 2A, the photodetection signal 8
When converting into a binary value, if a threshold value is set as 9, an output waveform 10 whose level changes from (0) to (1) as shown in FIG. 2B will be obtained. However, if the output light of the laser light source itself contains noise components, or if the light intensity changes due to a drop in laser output or dust, etc. in the optical path, the light spot may be However, since the times at which the threshold is exceeded are different, it becomes impossible to accurately detect the passing time. In other words, FIG. 3 shows that the time difference between the signal due to strong light such as 11 and the signal due to weak light such as 12 is t when the threshold value 9 is exceeded. ,
This may cause a decrease in image quality. The present invention aims to eliminate the transit time detection error that occurs when the light intensity fluctuates as described above.

In the present invention, as shown in FIG. 4, two photodetectors 16 and 17 are arranged close to each other in the optical scanning optical path. The light output after the light from the photodetector is scanned by the deflector 4 is as shown in 20 and 21 in FIG. 5 by giving an appropriate spread to the light beam diameter.
Differential operation is applied to the two photodetection outputs to produce an output as shown at 22, which is input to a comparator and converted into binary values. By doing this, the photodetection outputs 20 and 21 can be changed to the dotted lines 201 and 2 in FIG.
11, by setting the comparator threshold near 0, it is possible to accurately read the time at which the light spot passes through the midpoint 25 of the two photodetectors. In the present invention, the time at which light passes through the exact midpoint of two photodetectors is used as the reference time.

Next, if the bias 29 is set to 0 as shown in FIG.
It becomes indistinguishable from the difference between To avoid such drawbacks, if the output signal 301 of one of the detectors is biased and set higher than the threshold of the comparator, such as 30, and the output of the other detector is set to 31, then the differential output is indicated by 32,
The output 33 after binarization is as shown in FIG.
In this way, the difference between the output when no light hits the photodetector and the output when the two outputs are equal can be clearly distinguished. For light detection, it is sufficient to know the falling signal time.

Next, an example of an apparatus for implementing the present invention will be described. Consider a configuration as shown in FIG. 9 as an optical scanning position detector. 40 is a wedge mirror, and two optical scanning position detectors are arranged as 43 and 44. light 42
4 when passing over the mirror 40 from left to right.
The photodetection outputs of 3 and 44 are as shown in FIG.
The subsequent signal processing is as described above. This feature is effective in the sense that it can avoid the difficulty when it is not possible to arrange two photodetectors close to each other.

It goes without saying that, as a simpler case, the present invention can be implemented even if two photodetectors are directly arranged side by side at the position of the wedge-shaped mirror of this embodiment.

As described above, the present invention is very effective in that it is possible to accurately determine the position of a light spot when scanning with light, regardless of whether there is a variation in light output.

In addition, if it is impossible to place two photodetectors close to each other and they are placed apart by at least the diameter of the light beam, the output signal from the first detector is delayed appropriately, and the output signal from the second photodetector is It is possible to apply the present invention by temporally overlapping the output signal from the detector and extracting the difference component between the first and second detectors.

That is, as shown in FIG.
are arranged at a distance equal to or more than the diameter of the light beam, the output of the photodetector 61 is set to 71, and the output of the photodetector 62 is
Let the output be 72. If these two outputs are made into differential outputs as shown in 73, the two photodetection outputs do not intersect, so the midpoint between the two detectors cannot be found. Therefore, the output 71 is delayed by a predetermined time t ₀ by a delay circuit so that it appears like the output 74 and intersects with the output 72 in time. In this way, the two detector outputs are
It can be extracted as shown in the waveform shown in 5, 75
This makes it possible to set the time at which the output of the waveform shown by crosses zero as the reference time. That is, by setting the threshold value of the comparator near zero and using the output of the waveform shown by 75 as the input of the comparator, the reference time is determined by the output from the comparator.

[Brief explanation of the drawing]

Figure 1 is a diagram showing an example of an optical system for laser recording, Figure 2 is a diagram showing the output from one photodetector, and Figure 3 is a diagram showing the light intensity when using one photodetector. FIG. 4 and FIG. 5 are diagrams explaining the method according to the present invention. FIG. 6 is a diagram explaining the features of the present invention. FIGS. 7 and 8 are Diagrams explaining other methods of the present invention, No. 9, No. 1
0 and 11 are explanatory diagrams of an apparatus for carrying out the present invention. Explanation of the numbers, 1...laser, 2...modulator, 3...mirror, 4...polygon, 5...lens, 6...photosensitive drum, 7...optical scanning position detector, 10...binarized by comparator Output, 18... Optical deflector, 1
9...light, 29...bias, 31...detector output without applying bias, 32, 30 differential output of ₂ and 30 ₁ , 50, 51...two photodetector outputs.

Claims (1)

[Claims] 1. In an optical scanning position detection method for detecting the position of a scanned light spot, two photodetectors are arranged to detect optical signals from two positions on an optical scanning line, Extract the difference component between each output signal and calculate 2
By detecting the intersection position of the output signals of
An optical scanning position detection method characterized by determining a passing time of a light spot. 2. The optical scanning position detection method according to claim 1, wherein a bias is added to the output signal of one of the two photodetectors. 3. An optical element that divides the scanned light spot into two is arranged on the optical scanning line, and the two photodetectors are arranged at positions where the light beam divided into two is received by the optical element. An optical scanning position detection method according to claim 1 or 2. 4. Claim 1, characterized in that the output signal of one of the two photodetectors is applied to a delay circuit so that the output signals of the two photodetectors intersect in time. Optical scanning position detection method described in section.