IEEE Consumer Electronics Magazine - October 2015 - 74

Infrared

Ultraviolet

HUMAN VISUAL SYSTEM LIGHT
AND COLOR PERCEPTION

420

470

520

570

620

Wavelength λ (nm)

FIGURE 1. The full visible color spectrum and associated wavelength.

To better understand HDR technology, we need to carefully
examine how the human visual system (HVS) interprets color
and light information to shape our perception. The retina of
the human eye includes 130 million light-sensitive cells
called photoreceptors. Photoreceptors, via a process called
phototransduction, convert luminous energy (photons) to an
electric signal. This signal is transmitted through the optic
nerve and allows the brain to interpret the viewed scene. The
photoreceptors respond to the number of incoming photons
as well as their wavelengths. Naka and Rushton [2] fitted the
photoreceptors response as
R = In ,
R max
I n + vn

HDR is an emerging technology with its own requirements, challenges, and characteristics. This article focuses on
shedding light on the details behind the HDR technology,
including challenges and solutions related to capturing, transmission, and displaying.

(1)

where R and R max are, respectively, the neuronal and maximal
response of the photoreceptors to the grating stimuli I; v represents the semisaturation constant, meaning the value for
which half of the upper bound is reached; and n
is the photoreceptors' sensitivity. Figure 4 illustrates the photoreceptors' response at different n
Table 1. Common dynamic range terms and their applications.
and v values. This response corresponds to an
S-shaped
curve on a log-linear plot, with the
term
Formula
application
minimum equal to zero (because it is impossible
Orders of
Real-world, HDR imaging,
log 10 (Ymax) - log 10 (Ymin)
to have a negative occurrence of a phenomenon)
magnitude
e.g., three orders
and a maximal intensity. As is observed, n
Contrast ratio
Display, e.g., 1:1,000
1: Ymax /Ymin
impacts only the steepness of the slope while v
affects the location of the point of inflection.
Exposure latitude
Photography, e.g., ten
log 2 (Ymax) - log 2 (Ymin)
(f-stops*)
f-stops
Note that the photoreceptors' response cannot
cover the entire range of luminance that we can
Signal-to-noise
Digital camera, e.g., 60 dB
20 log 10 (Ymax) - log 10 (Ymin)
ratio (SNR)
perceive, unless the sensitivity (i.e., the steepness
of the slope) is drastically decreased. This is
*
f-stops: a difference in power of 2.
because although we can perceive luminance

Starlight

10-6

10-4

Moonlight

10-2

Overall Human Eye
Adaptation

100

Indoor

102

Sunlight

104

106

108 (cd/m2)

14 Orders of Magnitude

Human Instantaneous
Luminance Vision Range

Five Orders of Magnitude
Variable Range

Current Consumer
Cameras' Luminance Range

Three Orders

Conventional Consumer Displays'
Two to Three Orders
Luminance Range (Up to ≈ 500 cd/m2)

HDR Prototype Displays' Luminance
Range (Up to 10,000 cd/m2)

Four to Five Orders

FIGURE 2. The dynamic range of a real-world scene along with the capabilities of the HVS capture and display technology.
74 IEEE Consumer Electronics Magazine

october 2015

Table of Contents for the Digital Edition of IEEE Consumer Electronics Magazine - October 2015