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
CONTACT:
Midwest Optical Systems
www.midopt.com
8 July/August 2020 | Automation
automationmagazine.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46
