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Graefe’s Arch Clin Exp Ophthalmol
(2005) 243:26–32
CLINICAL INVESTIGATION
Herbert Jgle
Markus Pirzer
Lindsay T. Sharpe
The Nagel anomaloscope:
its calibration and recommendations
for diagnosis and research
Received: 18 December 2003
Revised: 3 February 2004
Accepted: 11 February 2004
Published online: 31 July 2004
Springer-Verlag 2004
H. Jgle (
)
) · M. Pirzer
Department of Pathophysiology
of Vision & Neuro-Ophthalmology,
University Eye Hospital,
Schleichstrasse 12–16, 72076 Tbingen,
Germany
e-mail: herbert.jaegle@uni-tuebingen.de
Tel.: +49-7071-2980744
Fax: +49-7071-294678
L. T. Sharpe
Department of Psychology,
University of Newcastle upon Tyne,
Newcastle upon Tyne, UK
Abstract Background: The Nagel
anomaloscope Model I is the defini-
tive clinical instrument for classify-
ing phenotypic variations in X-linked
color-vision disorders. Its system of
classification is based on the Ray-
leigh equation: the relative amounts
of red and green primary lights re-
quired to match a yellow primary.
Our aim was to characterize how
changes in mains voltage and ambi-
ent temperature influence the wave-
length and intensity of each primary
and alter the Rayleigh matches of
normal and anomalous trichromats.
Methods: A Nagel Model I anoma-
loscope was calibrated in wavelength
and intensity while varying the tem-
perature of its prism housing and the
mains voltage. Three normal, three
protanomalous and three deutera-
nomalous trichromats made Rayleigh
matches at various temperatures and
voltages. Results: The intensities of
the green and red primaries show an
exponential growth with mains volt-
age. Additionally, the wavelengths
and intensities of all three primaries
change with prism housing tempera-
ture. As a result, the R-G match
midpoints of normal and anomalous
trichromats shift with increasing
mains voltage, and more markedly
with increasing prism housing tem-
perature, to higher R-G settings.
Conclusions: Rayleigh matches ob-
tained with the Nagel I anomaloscope
are sensitive to changes in voltage
supply and prism housing tempera-
ture, arising largely from thermal ef-
fects of the internal light sources.
However, the instrument may still be
safely used for diagnostic and re-
search purposes provided that: (1) a
stable voltage supply is used; (2) it is
kept at a constant temperature; and
(3) the match midpoint of the refer-
ence population has been established
under identical conditions.
Introduction
The Nagel anomaloscope Model I, manufactured by
Schmidt & Haensch to a standard design until 1983, is
still the definitive clinical instrument for classifying the
phenotypic variation in X-chromosome linked color vi-
sion disorders [12]. Optically similar to an instrument first
described by Nagel [9], it is used to determine the Ray-
leigh equation; specifically, the match of a spectral yellow
light or primary to a mixture of spectral red and green
lights (the R-G match). It consists of a light source, a
separate internal adaptation light, an ocular, a compound
direct-vision prism and three entrance slits (see Fig. 1).
The entrance slits are carefully calibrated to define the
red, green and yellow primaries in terms of wavelength
and intensity, according to a norm (DIN 6160) of the
German Institute for Standardization [4]. However, the
accuracy of its calibrated settings is influenced by several
environmental factors, including temperature changes and
fluctuations in mains voltage, which have never been
fully investigated.
DOI 10.1007/s00417-004-0893-z
27
Mains voltage
It has long been known that the stability and reliability of
Nagel anomalocope matches are influenced by the mains
voltage [11]. Cavonius [2] found an increase of the R-G
match midpoints of normal trichromats with the mains
voltage of ca. 0.05 (Nagel) units per volt and concluded
that it is unlikely that these variations lead to clinical
misdiagnosis. However, although the nominal mains volt-
age was raised in 1983 from 220 V to 230 V in Germany,
no adjustment was made in the design or operation of the
instrument, which was constructed to run at 220 V. Fur-
ther, for reasons of harmonization within the European
Union, the permitted voltage range will be further ex-
tended to 195.5–253 V in 2004 [5, 6]. These changes
could potentially influence the Rayleigh matches and di-
agnosis of anomalous trichromats.
Temperature
Richter [10] was the first to report a seasonal variation in
the amount of red and green required to match the yellow
primary on the Nagel anomaloscope. Jordan & Mollon [8]
replicated his results on three anomaloscopes, including
two different Model I instruments (Schmidt & Haensch)
and an antique Model II instrument (manufactured by
Spindler & Hoyer). Expressing the matches in terms of
the anomalous quotient, a normalizing technique intro-
duced by Trendelenburg [13] to compensate for minor
changes in line voltage and bulb aging and to allow
comparisons between different instruments, they con-
cluded that the variation was almost certainly owing to an
instrumental sensitivity to ambient temperature—in par-
ticular, thermal changes in the prism—rather than to a
seasonal change in human physiology [8]. They estimated
the magnitude of the increase of the Rayleigh match
midpoint per degree Celsius of ambient room temperature
to be 0.175 Nagel units. However, they did not explore
the relevant sources of instrument temperature variation
in detail. These include not only thermal fluctuations in
external air temperature, but also changes in the heat
being emitted by the internal incandescent source (see
Fig. 1), located near the prism in the same housing and
used to provide a neutral adaptation light. The contribu-
tion of both of these temperature effects to the R-G
matches of normal and anomalous trichromats is yet un-
known.
Anomalous trichromacy
The classification of anomalous trichromacy is based on
the observation that protanomalous and deuteranomalous
observers require more of the red (protanomaly) or more
of the green (deuteranomaly) primary in the red–green
mixture to match the yellow primary than do normal
observers. The amount of red or green shift is a measure
of the severity of the anomaly. Given that the relative
amounts of red and green required in the match depend
upon the wavelength and intensity of each primary, and
that these, in turn, critically depend upon the lamp voltage
and the temperature of the instrument’s prism housing, it
is possible that voltage and temperature changes may
have a significant effect upon phenotype–genotype cor-
relations of anomalous red–green color vision. Therefore,
the principal aim of this study was to characterize the
wavelength and intensity changes of the Nagel Model I
anomaloscope primaries, arising from mains voltage and
temperature changes, and to estimate their effects on R-G
matches in normals and anomalous trichromats.
Fig. 1 Schematic diagram of
the Nagel Model I anomalo-
scope used in this study. The
diagram has been modified
from the original, which ap-
pears in the manufacturer’s
handbook
28
Methods
Subjects
Nine subjects were selected from a population of normal and
anomalous trichromats who were previously diagnosed by their
anomaloscope matches on the Rayleigh Model I instrument and on
the Oculus anomaloscope. They were classified as normal (MB, JA,
TG), deuteranomalous (RS, AS, SJ) or protanomalous trichromats
(TT, TZ, MP). The study was conducted in accordance with the
tenets of the Declaration of Helsinki and with the approval of the
ethics committee on human experimentation of the University of
Tbingen.
Apparatus and calibration
The Rayleigh equation settings were made with a Schmidt &
Haensch Nagel anomaloscope Model I constructed in 1979. All
physical (calibration) data were confirmed with a second instru-
ment constructed in 1980. Both instruments are equipped with a
220-V, 100-W cine-projection lamp for producing the red, green
and yellow primaries and a 220-V, 60-W incandescent bulb for
providing neutral adaptation. The instruments were connected to
the mains power supply through a variable transformer so that their
voltages could be adjusted between 100 and 250 V or kept constant
at 230 V. All voltages were controlled by a digital meter and
manually readjusted, resulting in residual fluctuations of less than
€0.5 V.
The emission spectra of the red, green and yellow primaries
were measured with a compact array spectroradiometer (CAS 140,
Instrument Systems, Munich, Germany). The half-field intensi-
ties—one half-field produces the yellow primary light, the other,
the mixture of the red and green primary lights—were measured
over the full range of R/G settings with a Pin-10 diode (United
Detector Technology, Santa Monica, CA) and a calibrated radi-
ometer (Model 80X Optometer, United Detector Technology).
To control the effect of the temperature on the R/G settings a
thermal sensor was mounted directly inside the prism housing, and
a second sensor was positioned 1 m away from the instrument on
the measuring table to monitor the ambient room temperature. To
reduce the instrument temperature below room temperature, the
prism housing was slowly cooled down to 15C by the appropriate
placing of Coolpacks (3 M “Nexcare” Coldhot Pack, 3 M Health
Care, D-41453 Neuss, Germany). Lower temperatures were avoid-
ed to prevent damage from water condensation within the prism
housing.
Procedure
Anomaloscope matches were determined in the preferred eye of
each subject. To avoid learning effects, all subjects were trained in
performing Rayleigh matches prior to the first day of experimen-
tation. In the mains voltage experiments, only normal trichromat
matches were made. They were averages of at least five settings at
each of 14 voltages from 120 V to 250 V. The order of voltages was
randomly chosen by the investigator. During the experiments, the
neutral adaptation light was turned on and the prism housing
temperature was maintained between 34C and 36.6C, with lower
temperatures corresponding to lower mains voltage.
In the temperature variation experiments, all matches were
performed at a constant mains voltage of 230 V. All experiments
were conducted with the internal neutral adaptation light switched
on. To determine the full range of acceptable matches the investi-
gator presented a series of R/G settings starting with the lowest
temperature. The subject was then allowed to adjust the brightness
of the yellow half-field, until an acceptable match was found or not
found with the R/G half-field. In the event that the half-fields were
seen as identical in color and intensity, the R/G setting and the
actual prism housing temperature were recorded. This procedure
was repeated at higher prism housing temperatures, ranging from
16C to 39C.
Results
Physical data
The physical calibration data, obtained at a prism housing
temperature of approximately 36C (see Methods), reveal
that the wavelengths of the green, red and yellow pri-
maries do not change with mains voltage. Further, at any
given temperature, the Nagel anomaloscope conforms to
the German DIN 6160, insofar as the calibrated primary
wavelengths and luminances fall within the standard,
accepted range of values. Nevertheless, the primary
wavelengths as well as the luminances vary systemati-
cally with mains voltage and prism housing temperature
so as to cause definable shifts in the Rayleigh matches of
both normal and anomalous trichromats.
Voltage data
The intensities (calculated for a 2-deg visual field) of
the green and red primaries (Fig. 2) grow approximate-
ly exponentially with the mains voltage. The significant-
ly higher increase in green primary intensity reflects
the color temperature change of the light source from
1821.5 K, at the lowest calibrated mains voltage of 100 V,
to 2292.2 K and 2512.9 K, at mains voltages of 196 V and
253 V, respectively (corresponding to the range permitted
in DIN EN 50160).
Fig. 2 The exponential growth of the red and green primary in-
tensities, in retinal illuminance (calculated for a two degree visual
field) with increasing mains voltage
29
Temperature data
The wavelengths of all three primaries change with prism
housing temperature. Two operational modes have to be
distinguished. When the internal neutral adaptation light
is switched off, the temperature stays approximately
constant at 24.5C for 15 min, then increases slowly until
it reaches its final temperature of 29C (Fig. 3A). Alter-
natively, when the internal neutral adaptation light is
switched on, after an initial 10-min period of nearly
constant temperature, the prism housing temperature in-
creases by about 12C, reaching its final temperature of
approximately 36.4C after about 90 min (Fig. 3B). Be-
cause the maximum temperature also depends on the
ambient room temperature, it may reach even higher
values. These data clearly show that the major source of
prism temperature variation is thermal variation (warm-up
effects) of the neutral adaptation light.
The temperature-dependent changes have two poten-
tial consequences for Rayleigh matches. First, the position
and width of the entrance slits defining the red and green
primaries (see Fig. 1) may be altered. Thus, the wave-
lengths of the primaries and the relative proportions of the
green and red primaries in the Rayleigh match may
change. Second, the refractive index of the prism and thus
the wavelengths of the green and red primaries may be
altered. As expected, our data (Fig. 4) show a shift of the
peak wavelength with increasing temperature from
549.7 nm and 667.0 nm, for the green and red primaries,
respectively, at 25.3C to 550.6 nm and 668.6 nm at
39.3C. These spectral shifts of 0.9 nm for the green and
1.6 nm for the red primary are clearly visible to both
normals and anomalous trichromats.
Rayleigh matches
Voltage data
As a result of the changes in the retinal illuminances of
the three primaries, the R-G match midpoints of all three
normal trichromats show a shift with increasing mains
voltage to higher R-G settings (i.e. requiring less green
and more of the red primary, see Fig. 5). Cavonius [2]
showed similar results obtained from two normal tri-
chromats and assumed a nearly linear dependency at
higher main voltage. We find, however, an approximately
exponential rise within the range from 190 V to 250 V.
From our data, we estimate the match midpoint of normal
trichromats to vary between 45 and 48 with a change of
the mains voltage from 190 V to 250 V. At the given
temperature of 36C, all values for the match midpoints
within this mains voltage range are higher than those
permitted by the DIN 6160 (36.5–43.8 units).
Temperature data
Figure 6 reveals, for two normal trichromats, three deu-
teranomalous and three protanomalous observers, how
R-G mixture settings on a Nagel Model I anomaloscope
vary with prism housing temperature. For each subject, a
series of matches were obtained within temperature ran-
ges centered at approximately 18C, 25C and 36C. Only
Fig. 3 Time-course of the change in room and prism housing
temperature, with the internal neutral adaptation light turned off (A)
or on (B)
Fig. 4 Dependency of the centroid wavelengths of the red and
green primaries upon the prism housing temperature. The wave-
length of the green primary shifts ca. 0.6 nm to longer wavelengths
per 10C temperature difference; that of the red primary, ca. 1.0 nm
30
the Rayleigh match determinations made at the two lower
temperatures by normal observers fall within the accepted
DIN 6160 ranges.
In the case of the anomalous trichromats, the width of
the matching range could be roughly estimated (see Ta-
ble 1). For all six, only a small range variation, between
ca. 1.5 and 3.0 Nagel units, was found, which did not
significantly change between low (18C) and high (36C)
temperature. (For the deuteranomalous subjects at 25C,
no range could be estimated because only a few matches
were obtained.) The anomalous quotients (AQ) have been
calculated using the average match midpoint of the nor-
mal trichromats obtained at the center of the given tem-
perature ranges. Based on these instrument- and temper-
ature-specific match midpoints, only small changes of the
AQ are observed for the three temperature ranges. How-
ever, usually the AQ is calculated using a fixed normal
match midpoint of 40 Nagel units, as suggested by
Zrenner [14]. If this is done, a significantly larger shift of
the anomalous quotient (AQ40) is found. For deutera-
nomalous observers, the AQ worsens (i.e., shifts away
from the normal value) with lower temperature; whereas
for the protanomalous observers, it improves (i.e., shifts
towards the normal value).
Fig. 5 The variation in R-G match midpoints for three normal
trichromats with increasing mains voltage. The solid lines represent
the best-fitting function (exponential rise to maximum) to the in-
dividual data
Fig. 6 Rayleigh-matches of two normal, three deuteranomalous
and three protanomalous observers on a Nagel anomaloscope as a
function of prism housing temperature. The continuous and dashed
curves represent the best-fitting linear regressions. The anomalous
quotient shown is based upon a normal match midpoint of 44
corresponding to a temperature of approximately 25C
Table 1 Rayleigh-matches of normal and anomalous trichromats
made at prism housing temperatures of the Nagel Model I anom-
aloscope within one of three ranges centered at 18C, 25C and
36C. Listed are the match midpoints (MMPs), ranges and anom-
alous quotients (AQs) calculated for each subject based on a normal
MMP estimated from the average normal matches at each tem-
perature. For comparison, the anomalous quotient for the highest
temperature range (36C), calculated for a normal MMP of 40, is
given as AQ40
Subject Eye Nagel 18C Nagel 25C Nagel 36C
MMP Range AQ MMP Range AQ MMP Range AQ AQ40
Normal
MB OD 41.20 0.5 0.99 44.40 0.5 1.02 46.43 1.0 1.04 0.69
JA OS 40.83 0.5 1.01 45.21 1.0 0.98 47.60 0.8 0.97 0.65
Protanomal
TT OD 63.25 1.5 0.16 63.75 1.5 0.20 67.00 2.0 0.13 0.09
TZ OS 62.75 2.5 0.16 64.75 1.5 0.17 67.25 2.5 0.10 0.07
MP OD 58.50 2.0 0.28 63.00 2.0 0.22 65.00 2.0 0.19 0.13
Deuteranomal
RS OD 24.00 3.0 2.79 27.50 – 2.71 29.75 2.5 2.91 1.95
AS OD 22.75 2.5 2.97 26.50 – 2.87 30.50 3.0 2.75 1.84
SJ OS 23.75 2.5 2.79 27.75 – 2.71 33.00 3.0 2.32 1.55
31
Discussion
We find a significant shift in Rayleigh match midpoints in
normal and anomalous trichromats with mains voltage as
well as with prism housing temperature, which should
be taken into account when establishing normal match
midpoints and determining anomalous quotients for red-
green color blind observers.
Main sources of voltage fluctuations in modern envi-
ronments are elevators and generators connected to the
same circuit. In regions with heavy industry or far from
the energy source, the average voltage may be smaller
than in our laboratory, in which a mains voltage range of
221 to 237 Volt was measured. However, even the small
voltage changes measured in our laboratory result in in-
tensity changes in the Nagel primaries that are clearly
visible to a sensitive observer and correspond to a shift of
ca. 0.5 Nagel units. These shortcomings could easily be
avoided by using a stabilized power supply.
Furthermore our results show a correlation between the
R–G mixture required to match the yellow primary and
the prism housing temperature. The shift is towards the
red primary (i.e., higher values) with increasing temper-
ature and is estimated as ca. 0.5 Nagel units per degree
Celsius. The total shift may be up to 10 Nagel units with
significant seasonal variations in room and instrumental
temperature. In terms of Nagel units, compared with a
normal match midpoint of 40, deuteranomalous observers
improve (i.e., shift towards the normal value) with higher
temperature; whereas protanomalous observers worsen
(i.e., shift away from the normal value).
Our data also indicate that the original recommenda-
tion of Trendelenburg [13] to use a anomalous quotient to
reduce variability between instruments is still satisfactory
from a clinical point of view. However, the unavoidable
temperature-dependent shifts make it impossible to have
normative data of the match midpoint available for all the
temperatures within the possible range of prism housing
temperature. Additionally, the temperature reached after
ca. 90 min (steady state) will also vary significantly with
ambient room temperature from the lowest value in
winter (ca. 18–20C) to the highest value in summer (ca.
36–40C), in rooms that are not climate controlled. The
prism housing temperature will be far less variable if the
built-in neutral adaptation light is switched off and a
separate light source is used for neutral adaptation.
Although changes in wavelength and intensity of the
primaries are mainly responsible for the shift of the match
midpoints, other physiological factors may contribute as
well. A reduction of the retinal illumination of the stim-
ulus, in addition to direct voltage and prism temperature
related changes, may affect the optical density of the cone
pigments and the R–G mixture required to match the
yellow primary. The size of such effects may vary among
subjects and may be genetically influenced [12].
Given the dependency of the Rayleigh match upon
prism temperature and mains voltage, it will have to be
determined in further studies whether the strict limits for
anomalous quotients set by regulatory authorities are fair,
consistent and reliable when using the Nagel Model I and
other mechano-optical anomaloscopes.
Conclusions
The severity of congenital red-green color-vision defects
is routinely determined using Rayleigh matches on tradi-
tional mechano-optical anomaloscopes. The most widely
used instrument is the Nagel Model I anomaloscope. The
matches obtained with this instrument are sensitive to
changes in voltage supply and prism housing temperature,
arising largely from thermal effects produced by the in-
ternal light sources. Additionally, these instruments were
originally designed for a voltage of 220 V and have not
been modified, even though the standard voltage in Ger-
many and Europe has been 230 V since 1983. While this
small voltage change results in a shift of the match mid-
point of normal subjects by ca. 1 Nagel unit, a difference
in prism housing temperature of 20C—as occurs under
the usual conditions of clinical investigations—results in a
shift of the match midpoint by ca. 10 Nagel units in the
direction of the red primary. Modern electronic anoma-
loscopes, which are not yet as widely used as the Nagel
Model I anomaloscope, overcome these problems by using
stabilized power supplies and color LEDs as sources for
the three primaries.
Its shortcomings notwithstanding, the Nagel Model I
anomaloscope may still be safely used for diagnostic and
research purposes, especially those involving the deter-
mination of the AQ in anomalous trichromats, provided
that: (1) a stable voltage supply is used (a modern UPS for
a single personal computer may be sufficient); (2) the
instrument is kept at a constant temperature (e.g., the
power is turned on at least 60 min prior to an investiga-
tion); and (3) the match midpoint of the reference popu-
lation has been established under identical conditions.
However, it will not fail to diagnose inherited red-green
color-vision defects when operated at any of the tem-
peratures or mains voltages within the ranges investigated
in this study. Nevertheless, special care has to be taken if
an expert opinion about the eligibility of a patient’s color
vision for certain occupations (including driver’s and pi-
lot’s licenses) is based on Rayleigh matches obtained with
such anomaloscopes.
Acknowledgements This work was supported by a grant from the
Deutsche Forschungsgemeinschaft SFB 430/A6 awarded to Herbert
Jgle.
32
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