Table Of ContentBiological Techniques Series
J. E. TREHERNE G. EVAN
Department of Zoology The Ludwig Institute for
University of Cambridge Cancer Research
England MRC Centre, Cambridge
England
Ion-sensitive Intracellular Microelectrodes, R. C. Thomas, 1978
Time-lapse Cinemicroscopy, P. N. Riddle, 1979
Immunochemical Methods in the Biological Sciences: Enzymes and
Proteins, R. J. Mayer and J. H. Walker, 1980.
Microclimate Measurement for Ecologists, D. W Unwin, 1980
Whole-body Autoradiography, C. G. Curtis, S. A. M. Cross,
R. J. McCulloch and G. M. Powell, 1981
Microelectrode Methods for Intracellular Recording and Ionophoresis,
R. D. Purves, 1981
Red Cell Membranes—A Methodological Approach, /. C. Ellory and
J. D. Young, 1982
Techniques of Flavonoid Identification, K. R. Markham, 1982
Techniques of Calcium Research, M. V. Thomas, 1982
Isolation of Membranes and Organelles from Plant Cells, J. L. Hall and
A. L. Moore, 1983
Intracellular Staining of Mammalian Neurones, A. G. Brown and
R. E. W. Fyffe, 1984
Techniques in Photomorphogenesis, H. Smith and M. G. Holmes, 1984
Principles and Practice of Plant Hormone Analysis, L. Rivier and
A. Crozier, 1987
Wildlife Radio Tagging, R. Kenward, 1987
Immunochemical Methods in Cell and Molecular Biology, R. J. Mayer
and J. H Walker, 1987
Radiation Measurement in Photobiology, B. L. Diffey, 1989
Radiation Measurement in Photobiology
Edited by
B. L. DIFFEY
Regional Medical Physics Department
Durham Unit
Dryburn Hospital
Durham DH1 5TW UK
f
ACADEMIC PRESS
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Copyright © 1989 by
ACADEMIC PRESS LIMITED
All Rights Reserved
Chapter 3 by T. M. Goodman © Crown copyright
No part of this book may be reproduced in any form by photostat, microfilm or any
other means, without written permission from the publishers
British Library Cataloguing in Publication Data
Radiation measurement in photobiology.
1. Photobiology
I. Diffey, B.L. II. Series
574.19'153
ISBN 0-12-215840-7
Typeset by Mathematical Composition Setters Ltd, Salisbury, Wilts
Printed in Great Britain by T. J. Press (Padstow) Ltd, Padstow, Cornwall
List of Contributors
L. O. BJORN, Department of Plant Physiology, University of Lund, Box
7007, S-220 07 Lund, Sweden
B. L. DIFFEY, Regional Medical Physics Department, Dryburn Hospital,
Durham DH1 5TW, UK
P. GIBSON, Glen Spectra Ltd, 2-4 Wigton Gardens, Stanmore, Middlesex
HA7 1BG, UK
T. M. GOODMAN, National Physical Laboratory, Tedding ton, Middlesex
TW11 OLW, UK
M. G. HOLMES, Department of Botany, University of Cambridge,
Downing Street, Cambridge CB2 3EA, UK
D. PHILLIPS, The Royal Institution, 21 Albemarle Street, London W1X
4BS, UK
M. SEYFRIED, Universit at Karlsruhe, Botanisches Institut 1, Kaiserstrasse
12, D-7500 Karlsruhe 1, FRG
A. W. S. TARRANT, Department of Chemical & Process Engineering,
Home Economics & Domestic Engineering Research Unit, University
of Surrey, Guildford, Surrey GU2 5XH, UK
A. D. WILSON, Applied Physics Group, Pilkington Optronics, Barr &
Stroud Ltd, Caxton Street, Anniesland, Glasgow G13 1HZ, UK
Preface
It is with pleasure that I write a preface to introduce this book on the
important topic of the measurement of optical radiation and its application
in photobiology. This volume arose out of a meeting of the British
Photobiology Society held in the historic buildings of the Royal Institution,
London, and was organized by Dr Brian Diffey.
The first three chapters are concerned with fundamental notions and
definitions, optical radiation detectors based on physical principles, and the
problems associated with calibration.
The next three chapters deal with important applications and extensions
of these radiant measurements, including a short review of biological and
medical users of lasers. As a dermatologist I admired and envied the
rigorous standards a botanist can apply to assessing action spectra in plants.
The final three chapters on specialized studies and developments illustrate
well the wide diversity that exists in photobiology. These cover ultraviolet
radiation dosimetry using polymer films, computer modelling of terrestrial
ultraviolet radiation and the "diffusion optics" in biological media.
Clearly the necessity of quantifying stimuli and responses is most
important in all branches of biology, particularly in photobiology. This
book I hope will stimulate interest and foster the best standards.
I. A. MAGNUS
Institute of Dermatology
London, UK
1
Basic Principles of Light Measurement
A. W. S. TARRANT
Department of Chemical and Process Engineering
Home Economics and Domestic Engineering Research Unit
University of Surrey
Guildford
Surrey GU2 5XH, UK
1.1. Light and Radiation — Introduction
Logically, one should talk about "radiation" first and then "light" as a
special case of it. But in this chapter I have chosen to discuss "light" first.
That is because we can readily visualize "light"; it seems simpler to picture
the various concepts in terms of light that we can see, and familiar lamps
that produce it, than to work in terms of unseen "radiation". To avoid
duplication the basic concepts are dealt with under the heading of "light",
so it is important that the reader who is not concerned with visible light
should not skip sections of this chapter.
This book is intended for biological scientists unfamiliar with mathemati-
cal and physical concepts. This chapter assumes no mathematical pre-
knowledge of the reader, and I must ask my colleagues of those disciplines
to bear with a lot of words rather than a few equations.
1.1.1. What do we mean by "light"?
Before we can measure light properly we have to be quite certain about what
we mean by "light". In common conversation we often use that word in a
very loose way, sometimes to the extent of talking about "ultraviolet light"
when what we mean is radiation that is invisible to the eye. So in scientific
terms what do we mean by "light"?
Imagine that we have a beam of light coming out of, say, a slide projector
(Fig. 1.1). For the time being let us assume that there is no slide in the
projector, so were we to shine this beam on a projection screen we should
just see a plain patch of white light. White light in fact is made up of light
of a whole variety of wavelengths, and if we make the light of different
RADIATION MEASUREMENT IN PHOTOBIOLOGY Copyright c 1989 by Academic Press Limited
ISBN 0-12-215840-7 All rights of reproduction in any form reserved
2 A. W. S. TARRANT
Fig. 1.1 A projector producing a defined beam of light.
wavelengths go in different directions by putting a prism in front of the
projector then we shall see that light spread out into a spectrum (Fig. 1.2).
The spectrum that we get will not appear equally bright in all parts. With an
ordinary slide projector the brightest part will usually be in the yellowish-
green part; as you go towards the red—longer wavelengths—it will become
less bright and ultimately will fade out altogether. If you go towards the
blue end—shorter wavelengths—from the yellow part it will get less bright
until it fades out in the violet. Now if we examine the part beyond the red
end we find in fact that there is radiation coming out of the projector and
falling there; we cannot see it, but we can detect it with physical detectors
such as silicon photodiodes. There may even be enough radiation there for
us to actually feel it—by sensing the heat it produces—on our hands. We
call this radiation "infra-red" radiation. Likewise if we look beyond the
violet part we find radiation coming out of our projector that we cannot see.
We can photograph it, and demonstrate its existence by making it cause
fluorescence.
If we could find out how much energy was coming out of our projector at
different wavelengths we would find it as shown in Fig. 1.3, with very much
more energy in the red and infra-red than in the blue or ultraviolet. Strictly
1. BASIC PRINCIPLES OF LIGHT MEASUREMENT 3
Blue Yellow
Green / Orange
200 400 600 800 1000
Wavelength (nm)
Fig. 1.2 The spectrum, with a rough indication of the colours seen.
5 r
er,
w
o
p
e
v
ati
el
R
0
400 600 800
Wavelength, X (nm)
Fig. 1.3 The relative spectral distribution of power in the visible region from a typical
projector incorporating a tungsten halogen lamp.
we should speak in terms of "power" coming out of our projector rather
than "energy", because we are concerned with the rate at which energy
comes out rather than energy as such. How is it then that the yellow or green
seems to be the brightest part?
The fact is that our eyes are differently sensitive to different wavelengths
—we are most sensitive to yellow-green light; less so to red and blue, and
not at all to infra-red and ultraviolet. Obviously if we are going to measure
"light" in physical terms we have to take this factor into account.
4 A. W. S. TARRANT
The relative sensitivity of the human eye to radiation of various
wavelengths has been much studied over the years. The usual method of
study involves asking human subjects to view a field in an optical
instrument of which one-half is illuminated with light of a known single
wavelength, whilst the other is illuminated with white light. The observer is
asked to adjust the brightness until they appear to be equally bright. This is
repeated for a series of different wavelengths throughout the spectrum, and
a set of mean results compiled for a group of several observers. The
sensitivity of any wavelength X, relative to that of the maximum sensitivity
at wavelength X x, is given by the inverse ratio of the amount required to
ma
match a constant white at X and X x. For example, if at a certain
ma
wavelength in the orange, say 610 nm, it requires 6 times as much power to
match the white as for the yellow-green of X x, then the eye sensitivity
ma
to radiation of wavelength 610 nm is obviously one-sixth of that of the
maximum.
In this way the curve representing the relative spectral sensitivity curve of
the human eye can be determined. It is usually spoken of as "the visibility
function", but it is officially called the "spectral luminous efficiency curve".
In some older books it is referred to as the "relative luminous efficiency"
curve. Notice that it is indeed only a relative sensitivity curve; it is only the
ratio of the sensitivity at any wavelength to that of the maximum.
Blue- Yellow-
Violet Blue green Green green Yellow Orange Red
V\
y,
c
n
e
ci
effi
s
u
o
n
mi
u
al l
ctr
e
p
S
350 400 450 500 550 600 650 700
Wavelength, X (nm)
Fig. 1.4 The internationally agreed curve for the spectral luminous efficiency function (the
"visibility function"). Reproduced from Henderson and Marsden (1972).
1. BASIC PRINCIPLES OF LIGHT MEASUREMENT 5
In practice the curves obtained by different individuals with normal
vision are closely similar. The standard curve is illustrated in Fig. 1.4, and
the internationally agreed standard data can be found in the British
Standards Publication BS 4727 part 2, and in any textbook on photometry
(e.g. Henderson and Marsden, 1972). The curve shown is that for normal
(photopic) vision. A different curve, also shown in Fig. 1.4, applies to the
dark-adapted eye (scotopic) vision.
1.1.2. The measurement of light
1.1.2.1. Luminous flux
Fortunately for our purposes the human eye in normal vision operates in a
strictly additive way; if it is presented with radiation of several wavelengths
simultaneously the response is simply the sum of the individual responses to
radiation of each wavelength concerned. We can use, therefore, the spectral
luminous efficiency curve to quantify "light"—that is our visual sensation
—provided we know how much power at each wavelength is emitted from
our source of radiation. If we divide that power spectrum into narrow
strips, as in Fig. 1.5, we can calculate the light-producing effect of each
narrow waveband, and add them up to obtain a measure of the total
amount of "light". Exactly how this is done is explained in the following
Pk
al,
v
er
nt
h i
gt
n
e
el
v
a
w
nit
u
er
p
er
w
o
p
e
v
ati
el
R —i 1—1 "11111 1 1—
400 600 800
Wavelength, X (nm)
Fig. 1.5 The spectral power distribution of a light source divided into a series of narrow
wavebands.
