Densitometer calibrated reference standard

A standard for calibrating a densitometer or the like is disclosed consisting of an optically transparent substrate supporting a plurality of spaced bands of different predetermined optical densities. Each band comprises alternate dark and clear strips and is sized to eclipse a beam of light generated in the densitometer and defining an illuminated zone. The width of the clear strips is large compared to the wavelength of the light beam and small compared to the width of the illuminated zone so that the quantities of light passing through the bands as the standard moves through the densitometer, and hence the optical densities of the bands, are directly proportional to the ratio of the areas of the clear strips to the areas of the dark strips in the bands. The bands are placed at an angle to the scan axis of the illuminated zone to present a gradient change in optical density to a scanning densitometer which is within the response time thereof. Additionally, the strips in each band are at an angle to the scan axis to pass across the illuminated zone with an apparent sweeping motion to average out any optical or illumination irregularities across the zone.

BACKGROUND OF THE INVENTION 
The present invention relates to the calibration of optical instruments and 
more particularly to apparatus for calibrating a densitometer or the like. 
The use of instrumentation such as the densitometer for measuring optical 
density is becoming more commonplace in many fields. For example, the 
assignee of this application has recently developed a dosimeter for use in 
monitoring the treatment of hyperbilirubinemia in infants by 
phototherapeutic means. The optical density (a measurement of the ability 
of a material to transmit light) of the dosimeter changes in response to 
the equivalent destruction of serum bilirubin in the bloodstream of the 
infant by light irradiation. Determination of the amount of bilirubin 
decomposed is dependent upon accurate measurement of the change in optical 
density of the dosimeter. Numerous federal and state agencies are 
requiring that means for certifying performance criteria be provided for 
instrumentation used in such medical diagnoses. 
Many materials have the ability to transmit or absorb certain frequencies 
of light. In the fields of compositional analysis, of which clinical 
medicine is an important part, this phenomenon has been put to good use. 
If white light containing the full spectrum of light frequencies is passed 
through a material sample, the light emerging from the sample will be 
modified according to the absorptive qualities of the material as to each 
of the frequencies of the light. Such "filtered" light then can be 
analyzed, and by comparison to known standards, the composition of the 
material or presence of specific components can be determined. In a 
typical scanning densitometer performing the above analysis, the sample, 
deposited as a thin film on a transparent substrate, is moved along a scan 
axis and across a zone of illumination in the densitometer. The light 
passing through the sample is detected and converted to an output signal 
which may be integrated to eliminate the effects of localized anomalies in 
the sample film. In this regard, the densitometer generates a chart trace 
of the output signal during the scanning operation together with a listing 
of integrated values of the output signal equivalent to areas under the 
chart trace. Such information enables a skilled technician to draw 
conclusions concerning the nature of the scanned sample. 
Of course, it is important that such densitometers be accurately calibrated 
and that such calibration be maintained during the operation thereof. 
Typically, the calibration and testing process comprises causing the 
densitometer to scan a standard presenting a known optical density 
equivalent and area. The response of a correctly calibrated and operating 
densitometer when scanning the standard can be anticipated. By comparing 
the output of the densitometer scanning the standard to the expected 
output, the performance of the subject densitometer can be determined and 
instrument adjustment made to bring the densitometer performance into 
correlation with the standard. 
In the past, such densitometer calibrating standards have been limited by 
the filter employed therein. In this regard, various filter types are 
available for incorporation into a standard. However, each has limitations 
in one or more areas of accuracy, linearity and cost -- particularly when 
it is desired to mass produce calibration standards for use with a widely 
distributed product line of densitometers or the like. 
Ideally, a filter for use in a standard should be accurate. That is, it 
should present an actual optical density equivalent equal to the intended 
optical density equivalent. Further, the filter should be producible at 
low cost in large quantities with uniform filter characteristics. Also the 
filter should be linear with wavelength. That is, it presents the same 
optical density equivalent regardless of the wavelength of the light 
passing therethrough. 
SUMMARY OF THE INVENTION 
The foregoing as well as other objectives have been achieved by the present 
invention in which bands capable of totally eclipsing an illuminated zone 
and comprising light attenuating strips in spaced relationship and 
displaying a known optical density of light passing therethrough are 
imposed on an optically transparent substrate. In the preferred 
embodiment, the bands are angled in relation to the densitometer scan axis 
and the strips comprising the bands are also angled in relation to the 
scan axis. The space between the strips is large compared to the 
wavelength of the light passing therethrough for detection in the 
densitometer and yet small in comparison to the width of the illuminated 
zone along the scan axis so that the amount of detected light can be 
converted by simple mathematics to an optical density equivalent for the 
band.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A densitometer standard 10 made according to the present invention is shown 
in FIG. 1. Densitometer standard 10 comprises a series of bands 12 of 
different predetermined optical density imposed on an optically 
transparent substrate 14. The substrate 14 may be any suitable homogeneous 
transparent material such as glass, plastic film, etc. The bands 12 may be 
photographically developed emulsions or chemically etched films such as 
chromium, and, ideally, should be as a thin film on the surface of the 
substrate 14 as shown in FIG. 2. 
The bands 12 of densitometer standard 10 are shown in greater detail in 
FIG. 3. The bands 12 are precision microimages of alternate dark 
light-attenuating strips 24 and clear strips 26. For best performance, the 
light-attenuating strips 24 should be of an optical density in the range 
of 3.3 or greater. If an illumination zone generated in a densitometer is 
imposed upon the bands 12 such that the illuminated zone is totally 
eclipsed within the overall perimeter of a band 12, then the amount of 
light which passes through the band becomes directly proportional to a 
ratio of the width or area of the light-attenuating strips 24 to the width 
or area of the clear or open strips 26. 
The operation of the bands 12 in the present invention is depicted in FIG. 
4. If the incident light (I.sub.0) 16 is passed through a band 12 having 
dark strips 24 and clear strips 26, the incident light 16 will be broken 
up into a number of small segments 22 indicated as I.sub.1 -I.sub.n. The 
effective transmitted light is given by the formula: 
##EQU1## 
The equivalent optical density is then expressed as: 
##EQU2## 
If the width of the clear strips 26 is large compared to the wavelength of 
light passing therethrough and small in comparison to the width along the 
scan axis of the illuminated zone, then the transmitted light can be 
converted by simple mathematics to percentage of incident light and, 
further, to an equivalent optical density according to the above 
equations. The dimensions of the bands 12 as manufactured, tested and 
hereinafter described have been carefully calculated to yield the 
following optical density equivalents: 1.0, 0.8, 0.6, 0.4 and 0.2. 
As depicted most clearly in FIGS. 1 and 3, it is desirable to angle the 
bands 12 (preferably between 30.degree. and 60.degree., measured forward 
or backward) relative to the scan axis of the densitometer and to angle 
the dark strips 24 and clear strips 26 in the same range in relation to 
the same scan axis. This creates sweeping action by both the bands 12 and 
the strips 24, 26 across the illuminated zone along a line normal to the 
scan axis so as to neutralize the effects of any irregularities in the 
illuminated zone and to ensure that the response time of the densitometer 
is not exceeded as it scans the standard of the present invention. 
In the later regard, it is common for scanning densitometers to include 
delay circuit for stabilizing the outputs thereof. Under such conditions, 
if a very rapid or step function change were to occur in the optical 
density of a sample being scanned, the output of the densitometer would 
change slowly over the response time of the delay circuits toward a new 
value corresponding to the new optical density of the sample. If the new 
value were achieved before the sample being scanned changed again in 
optical density, the densitometer output would be correct in quantity even 
though a slight delay between change and full response occurred 
internally. If, on the other hand, the optical density of the sample being 
scanned again changed before the new value was achieved, the output 
circuitry would attempt to achieve the value of the latest input without 
attaining the first value and the resultant densitometer output would be 
incorrect. In the latter case, the response time of the densitometer was 
exceeded. 
In the illustrated form of the standard of the present invention, placing 
of the bands 12 at a 30.degree. to 60.degree. angle to the scan axis 
causes the bands to sweep across the illuminated zone along a line normal 
to the scan axis rather than subject the densitometer to a rapid or step 
change in optical density. Such action may be appreciated by referring to 
FIG. 1 wherein a typical illuminated zone 28 is shown in relation to 
densitometer standard 10. As represented, as densitometer standard 10 is 
moved across the illuminated zone 28, each band 12 will move along the 
scan axis. If the illuminated zone 28 is perpendicular or normal to the 
scan axis as shown, as an angled band 12 moves across illuminated zone 28 
from left to right along the scan axis, the illuminated zone 28 will be 
gradually covered by the band 12 from the top down. As the band 12 
continues to move across the illuminated zone 28, the illuminated zone 28 
will remain totally eclipsed for a period of time and then be gradually 
uncovered from the top down. This gradual covering and uncovering or 
sweeping of the illuminated zone 28 along a line normal to the scan axis 
replaces the step function change in optical density created by a vertical 
band, with a gradient change in optical density within the response time 
of the desitometer. This assures that the response time of the 
densitometer will not be exceeded. 
With regard to possible irregularities in the illuminated zone, the source 
of illumination in a scanning densitometer is normally an incandescent 
bulb having a helically wound filament. This can create small differences 
in the amount of light at various points in the illuminated zone 28. 
Irregularities in the optical system creating the illuminated zone 28 can 
also cause non-homogeneous illumination across illuminated zone 28. If 
bands 12 moved across the illuminated zone 28 with strips 24, 26 parallel 
to the scan axis, only those portions of the illuminated zone 28 under 
clear strips 26 would be sensed. This could cause a slightly distorted 
response depending on the nature of the illuminated zone 28 at these 
points. By referring to FIG. 1, it can be visualized how, as the 
densitometer standard 10 moves across the illuminated zone 28 from left to 
right, the angle of the strips 24, 26 will produce a sweeping motion of 
the strips 24, 26 across the zone 28 from the bottom toward the top along 
a line normal to the scan axis in the same manner as the angle of bands 12 
produces a sweeping of the bands 12 from the top down as the illuminated 
zone 28 is covered and uncovered during the scanning operation. This 
apparent sweeping motion assures that all of illuminated zone 28 will be 
detected by the scanning apparatus of the densitometer (not shown) and not 
just selected portions. Thus, the apparent sweeping motion of strips 24, 
26 averages out any optical and illumination irregularities across the 
illuminated zone 28. 
In a densitometer it is common practice that the ambient or starting 
condition be set at a zero level. Then, if the scan of a sample begins and 
ends on the substrate supporting the sample, the optical density of the 
substrate is considered to be the ambient and is ignored. Under such 
circumstances, any change during the scan is a function of the sample 
alone and the media of the substrate does not enter into the calculations 
made by the densitometer. In the standard 10 of the present invention, the 
optical densities of the bands 12 present the only changes in optical 
density on the substrate 14 and are a function of geometry alone. 
Therefore, when the standard 10 is scanned by starting and stopping on the 
substrate, each band 12 presents a different yet constant optical density 
equivalent regardless of the wavelength of the light passing therethrough. 
Referring to FIG. 3 and FIG. 5, the construction of bands 12 as employed in 
a preferred embodiment of the present invention is shown in greater 
detail. In this regard, however, please bear in mind that while the 
dimensions and angles shown by FIG. 3 in conjunction with the accompanying 
table of FIG. 5 are preferred for a particular embodiment, other 
combinations of angles and sizes may be selected to meet different 
specific requirements of particular densitometers, spectrophotometers, 
colorimeters, or the like. In particular, the scan rate of the instrument 
determines the range of angles of the bands 12 which will prevent an 
exceeding of the response of the associated densitometer. In a similar 
manner, the angle of the band 12 and the dimensions of the illuminated 
zone of the instrument will determine the optimum angle for strips 24 and 
26 in sweeping across the illuminated zone. 
Further, the pitch or distance between dark strips 24 should be chosen to 
minimize distortion. In this regard, if a large pitch is employed, the 
effect is a series of bright and dark strips moving across the scanner. 
Moreover, since a glowing helical filament light source is most often 
employed in densitometers, if the pitch is large, the filament itself will 
appear as brighter and less bright areas within the clear strips 26. On 
the other hand, if the pitch is too small, diffraction effects of narrow 
clear strips 26 will become significant. In other words, some of the light 
rays will partially bend away from an angle of incidence normal to the 
scanner so that the light received by the scanner will not be according to 
the calculated value. Ideally, the pitch should not be less than 
approximately 10 times the wavelength of the incident light in order to 
eliminate significant diffraction effects. With incandescent light this is 
about 0.003 inches or 333 strips per inch maximum. 
The use of the standard 10 in checking the performance of a scanning 
densitometer can best be understood with reference to FIGS. 6 and 7 
wherein actual densitometer outputs produced by scanning a densitometer 
standard 10 as described herein are depicted. Referring first to FIG. 6, 
the output from an acceptable operating instrument is shown. Scanning the 
series of five bands 12 yields a chart showing five peaks with flat tops. 
The shape shows the gradual rise, period of eclipse, and gradual fall 
produced by the angle of bands 12. The slight ramps at the tops and 
bottoms of the leading and trailing edges are a function of the response 
time described earlier. 
Since optical density (O.D.) is the logarithm of 1/T where T is 
transmittance of light, if a logarithmic recorder is used and the 1.0 band 
is set to read 100%, densitometer linearity can be checked. On the chart 
of FIGS. 6 and 7, 100% on the chart corresponds to 1.0 O.D., 80% to 0.8 
O.D., 60% to 0.6 O.D., 40% to 0.4 O.D. and 20% to 0.2 O.D. As can be seen, 
when the densitometer producing the output of FIG. 6 scanned the standard 
10 of FIG. 1 having bands 12 of effective optical densities 1.0, 0.8, 0.6, 
0.4 and 0.2, the output was substantially linear as shown by the peaks at 
100, 80, 60, 40 and 20%. 
Next, since the size of the zone of illumination is fixed and if the bands 
12 are a very accurate and equal width, the areas traversed by the zone 
will be the same for each band 12. Weighing this area by the optical 
density equivalent for each band will then yield relative percent values 
for each of the bands. The relative percent values will be directly 
proportional to the areas under the curves on the scan chart. For example, 
for the standard of FIG. 1 used to produce the output of FIG. 6, the sum 
of the optical density equivalents of the bands (1.0 + 0.8 + 0.6 + 0.4 + 
0.2) equals 3.0. Thus, the portion of the intergrated total attributable 
to the 1.0 O.D. band is 1.0/3.0 or 33.33%. Likewise the 0.8 O.D. band is 
0.8/3.0 or 26.66%. In the same manner the remaining bands represent 20.00, 
13.33 and 6.66% respectively. The printouts of 32.0, 26.7, 20.5, 13.8 and 
06.9 represent the integrated values calculated by the densitometer and 
would be considered to be within acceptable limits. 
By contrast, refer to FIG. 7 which shows a densitometer output of 
questionable acceptability produced from the same standard. The change in 
output was caused by the insertion of an unclean optical filter. 
Maladjustment or malfunctioning parts may cause an even more pronounced 
deviation from the acceptable output of FIG. 6 which may be easily 
recognized as unacceptable and alert the operator to the need for 
corrective action before the instrument was used further. 
It is to be understood that while the alternate dark and clear strips 24 
and 26 are shown as straight, equally spaced, and parallel, this is the 
preferred embodiment thereof. Bands 12 comprised of curved or patterned 
alternate dark and clear strips providing a sweeping action across the 
illuminated zone also may be employed in the standard of the present 
invention.