The Human eye versus Silicon

Bart Dierickx, Caeleste, update 30sept2010

In the technical debate (the good one against the bad ones, or CDD against CMOS, or Silicon against Film) too often the single most important competitor is forgotten: the human eye. For fundamentalists the Human Eye [2] is the nec-plus-ultra, for contra-revolutionaries, it is one of our weaker parts that deserves an urgent bug fix. For all performance specs related to image rendering, it is the de facto standard.

In het next tables we collected the major spcifications of four competing imaging systems: the eye, the two major flavors of semiconductor imager technology (CCD and CMOS), and chemical film. We silently generalized all flavors of CCD (FT, IL, CS, CMD) and CMOS (diode array, passive pixels, pinned diodes…), and neglected some important outsiders, as camera vacuum tubes, photon counters, and the klingon eye.

Table 1: optical performance [1]

criterion	eye	CCD	CMOS APS	film
spectral response	400-700 nm peaked at 555	350-1000 nm	350-1000 nm	300-700 nm
peak quantum efficiency	<20%	>>50%	>50%	<10%
dynamic range	1000000:1 logarithmic 100:1 linear	10000:1 linear	6000:1 linear 100000000:1 logarithmic	10:1 ...100:1 non-linear sigmoid-like response 10000:1 by exposure latitude extension
dark limit	0.001 lux 1E-6 W/m^2	typ: 0.1 lux <0.0001 possible	typ: 0.1 lux <0.0001 possible	virtually zero
noise photons (*)[3]	10	typ:100 best:10	100	100
integration time or response timeconstant (room temperature)	0.02-0.5 s	33 ms typical 5 minutes possible	33 ms typical 2 minutes possible	virtually unlimited
max. frame rate	ca. 15 Hz	10 kHz	>> 100 kHz	1 shot only
speed of automatic illumination control	1 minute	none	typ. 1 frame	none

Table 2: optical resolution

criterion	Eye	CCD	CMOS APS	film
#pixels	120M cones	typical: 800K record: 90M (Fairchild)	typical: 800K…10Meg	typical: >1e6 grains, 10 to 0.01 grains/um2
pixel pitch	2-3 µm	2-10 µm	3-10 µm	effectively 10..20 um (grain size is not equivalent to pixel size!)
focal plane size	3 cm	<1 mm to wafer scale	<1 mm to wafers size	only limited by film size

Table 3: operating conditions

criterion	Eye	CCD	CMOS APS	film
radiation hard	1 mrad	10 krad to 100 krad	10 krad to 20 Mrad
operating Temp	36 C	-200 C...+200 C	0 K ... +200 C	0K ... 100 C
power dissipation	< 1mW	500 mW typ	50 mW typ	nihil

Table 4: on-board image processing

criterion	eye	CCD	CMOS APS	film
cosmetic quality	perfect by definition	very good	very good	as good as perfect
color	ideal hue gamut by definition	3-color acquisition	3-color acquisition	3-color print quality
absolute photometry	impossible	easy	easy	possible
focal plane processing	extensive	none	often	none (apart from DIR: development inhibitor releasing couplers)
access method	data driven (focus of attention)	serial only	serial, random access, ...	optical only
datapath	5M nerves	8-14 bits	8-14 bits	none

Table 5: logistics

criterion	eye	CCD	CMOS APS	film
price	invaluable	typ 20 EUR	typ 1 EUR	typ 0.1 EUR
2nd source	none	few	many	few
technology development cycle	500 Myears	3 years	1 years	20 years
fabs	3E9	5	10	5

Table 6: Eye, CCD and CMOS principles

Eye	CCD	MOS diode array	MOS active pixel	film
photo-chemical detection, transfer by neurons	multplexing of charge packets by lossless transfer	multiplexing by switches on photodiode node	multiplexing of the local amplifier output	photo-chemical detection and development
rod and cone signal levels are "processed" by ganglion cells	one charge sense amplifier at the end of the CCD register	one charge sense amplifier on the output bus	(charge sense) amplifier in every pixel	pure local change in optical absorption

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[2] the real name is the vertebrate’s eye. It’s not really a man made development.

[1] Bart Dierickx, “Silicon versus the Eye”, presented at the IEEE Workshop on CCD&AIS, 4 june 97

[3] noise photons is defined as: noise photons = noise electrons / (quantum efficiency * fill factor)