Duochrome tests

Used in conjunction with the crossed cylinder.

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'The red and green bit' on an optician's test chart can also be a separate instrument. Used in subjective refraction it would normally be known as a Ducochrome test but other titles meaning 'two colours' have been used at various times including bichrome test or, as here, bi-chromatic test. One of the colours is always red. The other may be green or blue. Whichever colour is used the test exploits the fact that when viewing a distant object the normal eye will focus on the yellow part of the visible spectrum, so an eye with impaired vision (or a miscorrected defect) may experience a form of chromatic aberration in the same way that old telescope lenses used to... and this may cause the patient to see more distinctly whatever lies on the green or red section of the test.

The target objects in this example are known as Verhoeff's Circles. Frederick Herman Verhoeff (1874-1968) was a famous American ophthalmic researcher and pathologist in Boston but in his early career his work concentrated upon optics, muscle balance and refraction. He was studying in London circa 1902. Verhoeff's family was of Dutch-German extraction and the thickness of the targets is modelled after the Snellen letter type devised in Utrecht in the 1860s. The thickness and overall diameter of the inner ring equates to a 6/6 Snellen letter. That of the outer ring equates to a 6/15 letter.

Freeman Bichromatic test

This particular version of the test was designed by Harry Freeman FBOA and manufactured on his behalf by the London firm of Raphaels Ltd. Note how the translucent colour film has started to degrade. In the 1940s Freeman became an advocate for reviving the crossed-cylinder technique and for the use of full or broken rings as target objects rather than Snellen letters. In the 1950s he built upon the work of another Fellow of the British Optical Association, Louis Cowen from Brixton, who had pioneered the use of polarised filters and visors with bichromatic tests as an effective test of binocular vision.

Presumably the lip on the end was for use when sliding the test into a receptacle on an illuminated light box. Did you ever use this instrument in your practice? We'd love to hear your memories:

Bo Hesselmark from Kvidinge, Sweden, an optometrist who qualified in 1964, wrote to tell us: I have been using this extraordinary device in my practices during many years. I got a scholarship from the Swedish Opticians to attend the London Course of Optometry. This course was run by Harry Freeman and Mr Flick and only foreign students were invited. Students from Europe and Asia attended. I am still in contact with the three students from Sri Lanka who attended and I have visited Sri Lanka many times since. After coming home I made a wooden box with lamps for illumination. I used this instrument in four different practices up to 1997. After closing my practice I sent all my instruments to Zanzibar, Tanzania where this box is still in use, I hope.

How it works

When looking through the lenses the optician has selected for me (or through a crossed cylinder instrument that he is holding in front of me) I see the target against the red background more distinctly: Either my myopia is under-corrected or my hyperopia is over-corrected. The optometrist must select another lens.

Now I see the target against the green background more distinctly. Either my myopia is over-corrected or my hyperopia is under-corrected. The optometrist must try again.

Now I see both targets with equal sharpness. The optometrist has discovered the proper correction for me.