Lighting & Vision

This is a response to the message posted by Terry McGowan (TKM) of
1/30/06

There are some technical errors and misconceptions in McGowan's
posting. The question of acuity improvements using LPS lighting or any
monochromatic source has been investigated by very prominent vision
scientists. One of the most definitive papers is 'The effect of
chromatic aberration on visual acuity' by F.Campbell and S.Gubisch (J.
Physiology 192: 345-358, 1967). The conclusions and implications have
been summarized by other well- known vision scientists in a more recent
review article 'The effects of ocular chromatic aberration on monocular
visual performance' by LN Thibos, A Bradley &Y Zhang. (Opt. & Vision
Sci, 68: 599-607, 1991). I quote from their article " In summary,
little improvement in visual performance for white light
should be anticipated by any attempts to correct for the chromatic
difference of focus of the eye. Furthermore, not only are the practical
benefits small, but when one attempts to achromatize the human eye, new
problems surface which can actually make the situation worse." Thus, if
best visual acuity is a lighting design imperative, it would be
erroneous to recommend LPS lighting. On the other hand there is a
simple and well-established method for such a lighting application and
that is to employ the highest CCT (correlated color temperature)
acceptable to the users. This result has been documented by repeated
studies by Berman et al and independently by Navvab published in the
peer reviewed Journal of the IES. McGowan implies that these studies
are controversial, but the results have never been found by anybody to
be incorrect. The controversy as raised by IES and CIE appears to be
whether they apply for general lighting practice beyond an application
that calls out for best acuity. The IES & CIE concern seem to based on
the their belief that in the workplace environment one never encounters
letters or objects small enough to be at a typical visual acuity
limit.Suppose that is correct. However that doesn't preclude that
visual acuity is not a useful means of evaluating the workplace visual
environment. I explain this in a few paragraphs below based on the
literature in
optometry and vision ergonomics. There is long history of work in
vision that relates visual acuity (the threshold condition) to reading.
The earliest work that I am aware of is the 1966 work of Flom (see some
references below) who introduced the concept of 'visual acuity
reserve'. Flom determined that reading speed slowed as readers
approached their threshold print size (letter
acuity): i.e., the deceleration in reading speed indicated that the
limit of the readers "reserve of resolving power" was approaching.
Since the 1980s other vision researchers (below are 4 references from
the vision and optometric literature and one from the J.IES) have
developed tests and conducted studies to determine the magnitude of the
visual acuity reserve for normally sighted people of all ages. Visual
acuity reserve for individual readers is expressed as the ratio
between the size of the smallest print that can be read with best
efficiency and comfort and the smallest size print that can be just
barely correctly read (acuity). The essence of this body of work is
that a persons reading speed will begin to slow down as letter size
decreases. The slowdown starts to occur when the letter size reaches
about 3 to 4 times the acuity limit and then begins an abrupt reduction
in speed as letter size approaches the acuity limit. This size where
the slowdown begins is referred to as the 'critical print size' The
region of letter sizes larger than
this critical size is referred to as the visual comfort zone and people
like to be in their comfort zone. For example, newspaper print is
generally 8-point type and at normal reading distance this corresponds
to 20/50 vision. A person needs to have just slightly better than 20/20
vision to be assured that they are in the visual comfort zone.
Newspapers are not printed with 3-point type although a person with
20/20 vision could read such type and thereby save much paper. The
reason is because reading at that size would not be in the comfort
zone. The better the acuity a person has the more reserve that person
has for remaining in the visual comfort zone with all visual tasks.
Thus a lighting spectrum that improves visual acuity will provide a
larger comfort zone. Since there is much material in the workplace of
newspaper size, the assurance of 20/20 acuity is not of irrelevance
there. A second and related point concerns posture. As letter size
approaches the visual comfort critical size, people will initiate
posture adjustments or even squint, i.e. they will lean forward or make
other positional/facial changes in the attempt to remain in the visual
comfort zone. Such adjustments introduce postural strains and possibly
physical discomfort if occurring over long periods. Therefore the
better the acuity the more postural and visual freedom. In addition to
the above considerations there is the beneficial effect of better
acuity related to the visual clarity of edges in the visual scene. Most
all objects have edges and boundaries and these will be seen more
sharply and clearly when acuity is best, as acuity is a measure of the
visual system's high spatial frequency (edge) resolution. I also
speculate another benefit of higher CCT lighting that is directly
related to the value of smaller pupils caused by such lighting. Smaller
pupils will reduce the eye's accommodative response
by increasing depth of field (see Cuiffredda reference below). Through
this effect, smaller pupils can contribute to lessening visual fatigue
associated with the constant and constrained (fixed) accommodative
requirement of computer work. These positive visual effects enhanced by
higher CCT lighting provide the circumstances for lighting practice to
improve the ergonomics of the workplace environment. To claim that
acuity and pupil size control is irrelevant to this environment invites
a missed opportunity beneficial to the quality of illumination
engineering.
References:
Flom, M.C. (1966). New concepts on visual acuity. Optometry Weekly.
57(28), 63-68.
Bailey, I.L. et al (1980). The design and use of a new near-vision
chart. Am J. Opt. & Phys. Opt. 57,740-753.
Legge, G.E. et al (1985). Psychophysics of reading I: Normal vision.
Vis. Res. 25, 239-252.
Whittaker, S.G., et al (1993) Visual requirements for reading. Opt.&
Vis Sc., 70(1), 54-65.
Bailey, I.L. et al (1993) Size as a determination of reading speed.
J.IES 22,102- 117.
Lovie-Kitchen, J.E. et al (1994) Th effect of print size on reading
rate for adults and children. Clnc & Exp Opt. 77, 2-7.
Cuiffredda, K.J. (1998). Accommodation, the pupil and presbyopia. W.J.
Benjamin (Ed). ' Borish's clinical refraction' (pp. 77-120)

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