FOCUS Machine Vision Filtersy

Count the squares By Barry Warzak, Founder, Midwest Optical Systems B

andpass fi lters are used when imaging to block unwanted ambient light, while passing only the necessary light output

to increase contrast and resolution. Interfering light is typically caused by the sun or overhead lighting in a building or factory. A bandpass fi lter will block this light without sacrifi cing the vision system’s monochromatic light. But not all bandpass fi lters function the same. The spectral output from a single colour LED light source used in any vision application typically follows a normal Gaussian, or bell-shape, profi le. For a machine vision bandpass fi lter to be most eff ective, the position, height and width of the passband should also emulate this bell- shaped curve. However, most fi lters on the market have a high, fl at transmission profi le. With this design, an overwhelming amount of ambient light is able to pass through at the weaker tail ends of the LED spectral output curve. Here’s a quick way to visualise the

diff erence between a MidOpt Bandpass Filter, which has a Gaussian profi le, and other bandpass fi lters:

Superimpose the spectral transmission curves for each fi lter over the spectral output curve of the monochromatic light. It’s most helpful if the graph has vertical and horizontal gridlines that can be used to divide the entire area of the graph into squares.

Looking at the graphs of the two fi lters,

the area of the graph (Figure 1) both fi lter curves can be ignored. The two fi lters are essentially performing the same in this region. Now pick one of the fi lters to analyse; see Figure 1.

First look at the LED output curve. Note

the area of this LED output curve that is outside of (above) the fi lter transmission curve. This represents the desired light

Figure 1: MidOpt Gaussian filter profile

that is lost. Count the squares in this area – estimating as necessary when there are fractions of a square; see Figure 1. Next, note the area of this same fi lter’s transmission curve that is outside of (above) the second fi lter’s transmission curve. This represents additional ambient light that is transmitted by only the one fi lter. As best possible, count the squares in this area. Add both of the above two square totals together. Record this number.

Next, follow the same steps, this time using the other fi lter. You will end up with a second number of total squares for this fi lter. Whichever number is lower indicates the fi lter that allows the least amount of ambient light and that will perform better by off ering the best contrast. To ensure maximum performance, the position, height and width of the passband should emulate the bell- shaped spectral output curve of the LED illumination being used.

Figure 2: MidOpt BP590 bandpass filter


Midwest Optical Systems

8 July/August 2020 | Automation

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