All standard optical choppers come with discs with a 1:1 mark to space ratio. This allows transmission of exactly 50% of the beam being chopped. However, there is often the requirement for the optical beam to chopped with a different mark to space ration.

This application note shows that transmission rates of less than 50% can easily be achieved with the Scitec Instruments range of optical chopper discs. Unfortunately, transmission rates of greater than 50% is more difficult as it requires a custom disc to be manufactured.

0% to 50% Transmission

A standard 10 slot disc, along with all our other standard discs, has a 1:1 mark to space ratio. This gives a transmission rate of exactly 50%.

If a second 10 slot disc is mounted on top of the first disc, and then rotated by 9°, the mark to space ratio is changed to 3:1. This give a transmission rate of 25%.

Obviously, increasing the rotation angle of the second disc will increase the mark to space ratio and reduce the transmission rate. In this case the rotation is 16.2°, the mark to space ratio is 19:1 and the transmission is 5%.

Please note that the slot width at the edge of the disc should be a minimum of 0.4mm to ensure that the opto-switch still operates successfully.

50% to 100% Transmission

Mounting two discs at once is a nice trick for reducing the transmission to below 50%. Unfortunately there is no simple trick to increase the transmission rate. The only solution is design and produce a custom disc which has the transmission rate that you require. As this technique reduces the strength of the disc, carefull design is required to ensure the disc can be run at speed. A number of sample designs are show below. Please note that the production of custom parts is expensive and generally has a minimum order quantity of 5 (for our 102mm discs).

Custom 30 slot disc with 75% transmission. Again, two discs can be mounted together to enable transmission to be varied between 50% and 75%.

Custom 2 slot disc with 98% transmission. Due to the tynes on the edge of the disc, designs of this sort should only be used within a blade protector.

0% to 100% Transmission with One Disc

This rather unusually shaped disc allows transmission from 0% to 100%. If the beam is small and positioned close to the centre of this disc then down to 0% transmission can be achieved. As the beam is moved further away from the centre of the disc, the transmission increases in linear fashion until 100% transmission is achieved at the edge of the disc. Again, this disc should only be used in some form of blade protector.