Optical Lens and sensor setup

  • Thread starter kdkdkd
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im in a slight fix.
i am presently using a light sensor. i was asked to use a lens along with it recently. is it because the obect i am going to sense is magnified by the lens so that sensor can detect it easily?

but, lets take the situation where a real image is formed because of the lens. Dont we need a screen in this case to project the image, for the sensor to view it? or is it that the sensor doesnt need a projected image.


lets imagine my sensor is a transmitter-detector led pair at an acute angle.
can a light sensor detect the converging(or diverging) rays from the lens, or do i require a proper projected image for sensing.

waiting for a reply.
 

Wrichik Basu

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lets imagine my sensor is a transmitter-detector led pair at an acute angle.
I am not quite sure what you mean by a detector LED. You can use an LDR as detector.
i was asked to use a lens along with it recently. is it because the obect i am going to sense is magnified by the lens so that sensor can detect it easily?
Trivially, a convex lens is a converging lens in all but one positions of the object. This means that even if your source gives light that is diverged and hence has low intensity per unit area, the use of a convex lens will increase the intensity to some extent. But this would highly depend on the position of your source and the relative position of the detector wrt the source. If the position of your source is fixed, it will be easier. Do some calculations in ray optics to find where the rays will converge,and position your detector accordingly. You can use a laser for your source, thereby eliminating the need of a lens. Further calculations will depend on where you're positioning the source, how large your detector is, and what the separation between the source and detector.
but, lets take the situation where a real image is formed because of the lens. Dont we need a screen in this case to project the image, for the sensor to view it? or is it that the sensor doesnt need a projected image.
That is not the case. I recommend you to read up something on ray optics for lens to better understand how things work.
 

Tom.G

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lets take the situation where a real image is formed because of the lens. Dont we need a screen in this case to project the image, for the sensor to view it? or is it that the sensor doesnt need a projected image.
The light sensor IS the screen. Position the lens to put the image on the sensor.
is it because the obect i am going to sense is magnified by the lens so that sensor can detect it easily?
A convex (positive) lens collects the light that shines on it and tends to aim it to a point. If you get the spacing between the object, the lens, and the sensor correct and also the focal length of the lens correct, the light collected by the lens will come to a point at the sensor. Since the lens is larger than the sensor, this results in more light on the sensor than the sensor by itself could collect.

Cheers,
Tom
 

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