Using Red and Green: In Black and White – Part Two
Filters vs. Targets
In Part One, we were introduced to a common activity in most VT rooms, which, of course, were the use of red/green filters. Just to summarize the main idea of that entry, with a non-lighted target on a white background, red cancels red and green cancels green.
It occurred to me, some halfway through writing Part One, that the distinction between filters and targets really becomes one of the more serious stumbling blocks for those of us using this equipment on a daily basis. So, here’s a question:
Assuming the patient is wearing red/green filters and both backgrounds are white, will a red printed target produce the same results as a red acetate overlay placed on top of a Hart Chart?
Ostensibly, the two scenarios might appear interchangeable. However, they are quite different.
To begin Scenario One, here are a few illustrations to assist your thought process:
Here in Picture One, we see one of GTVT’s letter charts which contains both red printed letters and black printed letters contained in red squares. Even for someone with a high amount of amblyopia or suppression, this target appears relatively unchanged. You could even cover one eye without the appearance of the chart changing.
Picture Two shows the chart when viewed through a red filter. As we can see, the printed red letters seem to have disappeared – remember, in this scenario red cancels red – but the remainder of the chart is clearly in view. This is principle is very similar to the ‘Red Racetrack” activity described in Part One, where the eye covered by the green filter is the only eye capable to producing input.
As an aside, if you’re ever wanting to see technology at it’s finest, try using a ‘red eye reducing’ camera to take pictures through a red filter. The camera doesn’t know what to do! 😛
Here in Picture Three, which was taken from behind a green filter, we can see that the red printed letters are now visible and the letter surrounded by the red boxes are no longer distinguishable.
In summary, these pictures are all examples of red printed targets when viewed through a filter. The takeaway message being that red cancels red.
Now, check out Scenario Two:
Here is a picture of a near reading card with red/green BAR’s laid above it. Directly below is a picture of the same chart when viewed through a red filter. Notice anything?
That’s right. The red lines are clear and the green bars have turned black. In this scenario, red sees red, which for those keeping score, is the opposite effect as the first setup. For good measure, below is the same chart from behind a green filter.
So why is this? What’s the difference?
There is a complex neurological reason for this, which frankly, confuses me every time someone attempts to explain it to me, so let’s keep it simple.
When you’re working filter to colored target, as in the Scenario One, corresponding colors cancel and the opposite eye becomes the one receiving the input. When it’s filter to filter, as in Scenario Two, corresponding colors work together and the opposite color is cancelled out, because filter to filter mirrors the effect of a lighted target.
Remember our motto – Cura te ipsum (Latin for ‘Heal Thyself’) – if you’re not sure, try it our yourself to be sure it’s producing what you are hoping for!
Stay Tuned for Part Three: Factoring In The Black