The contrast sensitivity function 資工三 黃明進 資工三 李啟毓 資工三 邱彥霖

Introduction  Some objects may not be visible at all because their contrast is too low. The invisible objects are said to be below some threshold. The visible objects are said to be above some threshold.  Ex. Driving a car in a very thick fog. Can you see the black writing on a white billboard ?

Black-White edge  Black has low intensity.  White has high intensity. Black-white image Represent edge graphically

Grating (repeating bars)  High contrast grating: light bars have a very high intensity and dark bars have a very low intensity.  Low contrast grating: light bars are not very light and the dark bars are not very dark. Same average

Grating (repeating bars)  Changing average intensity  makes average luminance either lighter or darker.  Changing contrast leaves the average the same  makes the blacks blacker and the whites whiter.

Grating (repeating bars)  Square wave gratings: the luminance lines go straight up and down and are flat on top.  Sine wave gratings: the luminance lines change in sine wave fashion.

Sine wave grating  Grating of different contrast but the same average luminance.

Definition of contrast  Definition of contrast:  (L MAX - L MIN ) / (L MAX + L MIN )  L MAX : maximum luminance  L MIN : minimum luminance  High contract: approximate 1.0.  Low contract: approximate 0.

Sine wave grating with one frequency  Contrast decreases as the picture goes up. Less sensitive More sensitive

Sine wave grating with varying frequency thick Harder to see Thin Harder to see More sensitiv e

Spatial Frequency  High spatial frequency- Lots of bars across particular distance  Low spatial frequency- few bars across particular distance Thin bars Thick bars

Spatial Frequency  Too obscure … Depend on the distance you look, because the more closely you look, the lower spatial frequency it will be

Visual angle  Definition- an object 1 cm tall at a distance of 57 cm subtends a visual angle of 1°

Easy way of visual angle Because one arm is about 57 cm long and a fingernail is about 1 cm in diameter, your fingernail at arm's length subtends about 1° So this spatial frequency is one cycle per degree of visual angle. One black bar + one white bar = one cycle

Contrast, Sensitivity, Frequency

Human vision  The most sensitive spatial frequency is about 2-5 cycles per degree for humans  the more sensitive you are the less contrast you need to see the object, as the following figure shows.

Extra information  Channel: A group of cells or neurons of human visual system There are many “ channels “ in human visual system Every channel responds to small ranges of spatial frequency  CSF is the "envelope" which just covers the peaks of all these individual channels.

Build your own contrast sensitivity function (CSF)  Gabori Attack It ’ s like a shooting game to measure the contrast sensitivity function Demo