The Bandpass Filter Selection Process and Application:
A Bandpass Filter is a device which passes frequencies within a certain range (band) and filters out frequencies outside of the given range (band). In machine vision terms, a Bandpass Filter is generally used to maximize contrast between features of an image.
As most machine vision cameras are monochrome, we will use a monochrome camera in this application. Monochrome cameras assign each pixel a intensity score of 0 to 255. Monochrome cameras are popular because they are usually cheaper, require less processing power, and therefore can image faster. Monochrome vision tools also tend to usually more robust than vision color tools as well.
As seen in the figure above, a pixel intensity value of 0 is black (absence of light) and a pixel intensity of 255 is white (presence of light).
Another important concept to understand is the color spectrum, we will focus on the visible color spectrum.
The color of visible light depends on its wavelength. Each color has a different wavelength. Red has the longest wavelength, and violet has the shortest wavelength. When all the waves are seen together, they make white light.
As we can see in the image above, different wavelengths correlate to a different color of visible light. For our purposes, we will focus on the main wavelengths of light available from machine vision light suppliers like Smart Vision Lights
|Smart Vision Lights Available Light Colors|
Lets imagine we are using a Monochrome Camera and are trying to pick out any components that are not the correct color (in this case we want to find components that are not red). We will also assume we are using a white light, which is comprised of all wavelengths of visible light.
First, lets look at an original image of the products we will be working with (This is taken with a color camera, although we will be working with a monochrome camera for this application).
We have successfully selected a bandpass filter to help improve our image contrast!
Please note: This article assumes the application is utilizing a white light. This application would also see success if used with a light that is the same color as the features we want to appear bright. For example if you are trying to make red items appear bright, then use a white light or a red light and a filter that passes the red light. If you wanted to do the opposite and make blue appear to be bright, then you could use a white or blue light with a 470nm bandpass filter.
If you still require assistance choosing a bandpass filter, feel free to contact our tech support team at (331) 684-7466 or Support@MachineVisionDirect.com