Graphics Reference
In-Depth Information
10
−
19
J, we see that such a bulb
converted to visible light. Dividing 2 J
/
sec
by 3
×
10
18
visible photons per second. An office—say, 4m
emits about 6. 6
×
2.5m—together with some furniture has a surface area of very roughly 100m
2
=
1
×
×
4m
10
6
cm
2
; thus, in such an office illuminated by a single 100W bulb on the
order of 10
12
photons we arrive at a typical square centimeter of surface each
second.
By contrast, direct sunlight provides roughly 1000 times this arrival rate; a
bedroom illuminated by a small night-light has perhaps 1
×
100 the arrival rate.
Thus, the range of energies that reach the eye varies over many orders of magni-
tude. There is some evidence that the dark-adapted eye can detect a single photon
(or perhaps a few photons). At any rate, the ratio between the daytime and night-
time energies of the light reaching the eye may approach 10
10
.
/
Because we also work with computer displays, and the computers driving these
displays typically draw polygons on the screen, it's valuable to have some num-
bers describing these. A typical 2010 display had between 1 million and 1.5 mil-
lion pixels (individually controllable parts of the display
6
)—which will soon grow
to 4 million pixels; with displays that are 37 cm (about 15 inches) wide, the diago-
nal distance between pixel centers is on the order of 0.25mm. The dynamic range
of a typical monitor is about 500:1 (i.e., the brightest pixels emit 500 times the
energy emitted by the darkest pixels). The display on a well-equipped 2010 desk-
top subtended an angle of about 25
◦
at the viewer's eye.
The human eye has an angular resolution of about one minute of arc; this
corresponds to about 300mm at a 1 km distance, or (more practical for viewing
computer screens) about 0.3mm at a 1m distance. When pixels get about half
as large as they are now, it will be nearly impossible for the eye to distinguish
them.
7
A one-pixel shift in a single character's position on a line of text may be
completely unnoticeable. Furthermore, the eye's resolution far from the center of
the view is much less, so pixel density at the edge of the display screen may well be
wasted much of the time. On the other hand, the eye is very sensitive to motion,
so two adjacent pixels in a gray region that alternately flash white may give an
illusion of motion that's easily detectable, which might be useful for attracting the
user's attention.
The lens of a modern consumer-grade digital camera has an area of about 0.1 cm
2
;
suppose that we use it to photograph a typical 100W incandescent bulb, filling
the frame with the image of the bulb. To do so, we place the lens 10 cm from the
6. Each display part may actually consist of several pieces, as in a typical LCD display
in which the red, green, and blue parts are three parallel vertical strips that make up
a rectangle, or may be the result of a combination of multiple things, like the light
emitted by the red, green, and blue phosphors of each triad of phosphors on a CRT
screen.
7. This doesn't mean it won't be worth further reducing their size; 300 dot-per-inch (dpi)
printers use dots that are about 0.1 mm, and their quality is noticeably poorer than that
of 1200 dpi printers, even when viewed at a distance of a half-meter. Distinguishing
between adjacent pixels and detecting the smoothness of an overall image are evidently
rather different tasks.
