Visual Performance

1. Visual Performance • Weston, and Blackwell, in the 50’s and 60’s investigated • the link between: • Size • Contrast • Illumination • If you decrease one, you can often compensate by increasing • another. For example, if you look at grey printed letters on a • white paper background, rather than black letters, you can • improve your visual performance by raising the ambient • illumination level.

2. Visual performance (spatial vision) Visual Performance C) rise in performance E) possible decline A) below threshold D) plateau 100% 0% Parameter (Size, Contrast, or Illumination) B) threshold

3. The Performance Model (consider size’, or ‘visual angle’) Consider what happens when you approach a movie poster: A) When you are too far away from it, you can’t read any of the text B) You reach a point where you can just make out some of the letters C) As you get closer you can read the Film title (written in the largest letters) but nothing else on the poster. When you get a little closer you can then read smaller writing - e.g. the name of the star, and when you get closer again you can read the names of the other actors. D) At some point you can read all of the poster, and moving closer does not improve performance E) When you get really close to the poster, you’ll find that you can no longer read the largest letters - the film title - but you can still read the smaller writing. Visual performance here has declined.

4. V F E U P E T P K V N U F C O U T F N P E V N L B Contrast Chart: Note that, here, the letters are all the same size

5. The following 4 slides illustrate the model when we are considering contrast. Go through them, and see how your visual performance changes.

6. P H E V E T P N U F C O K E V U T F N P E V N L B P U Chart 1

7. E H F E T P N U F C O K P L N F V U L L B E V N P U Chart 2

8. E H F E T P N U F C O K P L N F V U L L B E V N P U Chart 3

9. P H E V E T P N U F C O K E V U T F N P E V N L B P U Chart 4

10. Most people will see a large change in visual performance between charts 1 and 2, and little change between the others. In fact, the contrast change between the charts is similar. The following graph illustrates that the large change in performance noticeable between graphs 1 and 2 comes about because these are on the steep portion of the curve, whereas the others are near the plateau.

11. Visual performance (spatial vision) Visual Performance 3 4 2 1 100% 0% Contrast Chart: 1 2 3 4

12. Practical Issues (1): Disability glare If you are viewing this on a VDU, try shining a light onto the screen (producing disability [veiling] glare) and see which of the 4 letter charts are most affected. [which would you expect to be most affected?] Practical Issues (2) : 99HUP106/99HUC106 If you are looking through a car windscreen that is dirty - thereby reducing the amount of light reaching the eye, AND reducing the contrast of the object - which portion of the curve should you be on? This illustrates a general principle, often overlooked in practical vision research. To assess the change in performance produced as a consequence of the introduction of a variable (in this case a dirty windscreen) you need to use a test that is sensitive to small changes - and to do this you need to be on the steep portion of the curve. If you are on the plateau, (e.g. chart 4) then a small reduction in contrast is NOT going to alter visual performance by a noticeable amount. Hence the use of high contrast charts for practical vision research is generally to be deprecated!