The College of Optometrists

If asked in a case record pqe

why do you use a red target?

why do you use a 20mm target?

what would be your ideal size and colour and why?

what would you say?

red instead of white maybe because its better contrast i.e. doesnt fuse with the background (if youve got white walls, wear a white coat etc.)

a 15mm red target will give you the same isopter as a 3mm white target, due to the difference in distribution of the photoreceptors.

Red target:

Colour conduction defects

Optic nerve anatomy

Centro caecal loss

Retro bulbar optic neuritis

etc.

Quite frankly it doesn't really matter what you use as long as it has sufficient contrast and is of just sufficient size to stimulate a response in a 'normal' person from the extent of the peripheral, gross visual field, through to fixation.

There is a lot of dogma around about this and it makes me smile (as an examiner) when everyone trots out the same 15mm red target without knowing why they have chosen to do so.

You can do the angular subtense calculations as easily as the next man, and realise what % of the total gross fields you are sampling (hint - it is not a lot). However, at least you are attempting to measure something other than that famously poor test of macular function - refraction !!!

The technique is exactly what it says on the tin, 'Gross perimetry'. How often do you see your ophthalmologist scramble to use his 15mm red !!!!! Ask yourself why and why not?

I don't think my colleague does this question justice.

red is supposed to be sensitive for optic nerve pressure or optic neuritis. This is old research and may not be current thinking - as it has been superceded by threshold auto testing.

the point of confrontation is really to examine the width of the periperal field. This would be useful in ret det or gross loss as in hemianopia or optic nerve pressure etc. So it is a useful technique. Not sensitive to the finest nuance - but handy for its own use and quick and sometimes the only practical way of doin it.

The target has to be big enough to be seen in the background clutter and yet small enough to give some practical use. Practice makes perfect! In my PQE's I made a few of diferent size and color - and enjoyed the tests I did. Nowadays, I tend to use a pen top - which would be about 10mm. Usually red - but I also use green for a change. I can do this as I know the difference in result from repeated testing.

What I am saying - is practice and find out for yourself. It was developed by generations of optoms to help themselves work properly and has stood the test of time.