An optional fourth primary permits tetrachromatic investigation, and is useful for mesopic colour matching. We have been using this device for two years in our MSc Colour Imaging programme and have had very good results.
Students learn a lot by hands-on colour matching.
Results are not of national metrological accuracy, but are better than would normally be achieved by lab demonsration equipment. The binocular arrangement is easier to use than haploscopic techniques, and makes observer metamerism very apparent. Arthur Tarrant gave a paper on the device at the recent Colour 2000 conference at NPL in London. I am looking for a comercially available visual colorimeter. If anyone has any suggestions, I'll be deeply grateful.