Self aligning substrate transmittance meter

A portable substrate transmittance meter includes a remote transmitter and a remoter receiver for measuring a light transmittance of a substrate that does not have an edge available for sliding a base meter thereover, for example a fixed window on a vehicle. The remote transmitter is releasably attached to one side of the window with a donut shaped magnet resting thereagainst. The remote transmitter includes a light transmitter for emitting a predetermined light signal through a center of the donut shaped magnets and on through the fixed window. The remote receiver is place on the other side of the fixed window opposite the remote transmitter. Donut shaped magnets of the remote receiver cause the remote receiver to attract towards the remote transmitter in proper alignment therewith. A light receiver of the remote receiver receives the transmitted light for effecting a transmittance measurement of the fixed window.

FIELD OF THE INVENTION 
This invention relates in general to the field of light transmission 
measuring devices, and more particularly, to a meter for measuring the 
transmittance of a substrate. 
RELATED APPLICATIONS 
This patent application is related to pending patent application Ser. No. 
08/185,039, filed Jan. 24, 1994 and assigned to the same assignee as the 
present application. 
BACKGROUND OF THE INVENTION 
There have been available over the years many apparatuses for measuring 
opacity or transmissivity of materials. Such apparatuses have proven 
useful in production areas where it is necessary to monitor not only the 
presence or absence of objects, but further to more accurately measure 
aspects of a material relating to its opacity or transmissivity. For 
example, an apparatus that measures opacity of a material can be used for 
determining the thickness of such material, which information can be used 
for quality control. 
Still further, an opacity measurement apparatus is provided for determining 
a density of an effluent flowing in a furnace stack as described in U.S. 
Pat. No. 4,076,425, issued to Saltz. This apparatus uses a first light 
path directed through the effluent and a second light path external to the 
effluent. A ratio is determined from measurements taken from the first and 
second light paths for indicating the opacity of the effluent. Such 
apparatuses as described by Saltz, however, include several reflection 
mirrors and lenses and can be relatively cumbersome, complex, and 
sensitive. 
Suzuki et al., in U.S. Pat. No. 5,231,576, teaches an apparatus for testing 
specimen concentrations (blood) by reflecting a light off of the specimen, 
which apparatus requires a 60 second delay after the switch 36 is enabled 
by loading a test piece. Brunnschweiler, in published patent application 
GB 21772102A, teaches a light transmission measuring system using a second 
detector 14a for controlling the intensity of the light source. 
Automobile window tinting presents a field where it has become necessary to 
determine the transmittance property of the tinted glass. Such 
transmittance measurements are made ever more urgent because several 
states have passed legislation regulating the allowable transmittance of 
automobile windows. In some localities, enforcement agencies must resort 
to subjective tests in determining whether a given window is tinted too 
dark. For example, one known method of checking a window tint is 
accomplished by viewing a specially marked card through the window while 
making a visual check of visible patterns printed on the card. Enforcing 
such recently enacted legislation, however, requires a more scientific, 
accurate and repeatable measurement. For practicality purposes, this 
requires a portable, battery operated, transmittance meter having accurate 
measurement capabilities. Such a meter must not only be rugged, but also 
able to maintain accuracy so that any measurements taken will meet minimum 
evidentiary requirements in court. 
One such attempt of providing a portable window transmittance meter, for 
use by law-enforcement personnel, for example, is described in U.S. Pat. 
No. 5,073,707 issued to Marcin. The apparatus described therein includes a 
housing having a slot which may be slipped over a window pane. A switch 
detects the initial entry of the window pane into the slot which almost 
immediately thereafter causes a light emitting diode to transmit light 
that is measured and stored. Since the window pane has not yet fully 
entered the slot, the transmittance of air is initially measured for use 
as a reference. A second light measurement is made approximately three 
seconds later when the window pane is assumed to be fully inserted. A 
ratio is determined between the first reference measurement and the second 
measurement for providing an indication of the window's transmittance. 
Notwithstanding Saltz and Marcin, there still exists a need to improve both 
the reliability and accuracy of the transmittance meter. Specifically, 
Saltz's apparatus is inappropriate for portable use, and Marcin's meter 
may be affected by stray light, and may further produce false readings due 
to improper use, for example, from light scattering when the window is not 
properly inserted or when inserted too slowly (or moved after insertion 
but before the second measurement). The light scattering errors can occur 
from light scattered off the edge of the glass. Stray light should be 
accounted for and substantially eliminated for highly accurate 
measurements. Still further, inaccuracies may occur due to environmental 
conditions and circuit tolerances. 
These problems are further exacerbated by the need to test those windows 
that are not accessible for sliding a portable window transmittance meter 
thereover. For example, a stationary back window or a rear window may be 
tinted at a different darkness level than the other movable windows. None 
of the heretofore known portable meters provide a capability of placing a 
transmitter and detector on opposing sides of such a stationary window. 
Other problems present themselves in such a situation, for example, even 
if transmitters and receivers are so placed, they must be accurately 
aligned, a task that can be made especially difficult given limited arm 
lengths. 
Thus, what is needed is a portable window transmittance meter having a 
capability to place a transmitter and receiver on opposing sides of a 
substrate not having an open edge available, the transmitter and receiver 
being capable of self alignment so as not to depend upon an accuracy of 
manual alignment accuracy for such tests. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
substrate transmittance meter capable to measure transmittances on a 
substrate lacking an accessible edge. 
Another object of the present invention is to provide a substrate 
transmittance meter capable to measure transmittances on a substrate 
lacking an accessible edge, such measurement easily performed by one 
operator. 
Still another object of the present invention is to provide a window 
transmittance meter capable of measuring transmittances of stationary 
automobile windows, wherein an independent transmitter and receiver of the 
transmittance meter are self aligning. 
According to a first embodiment of the present invention, a method for 
measuring a percent transmittance of a mounted substrate is provided. A 
remote transmitter houses a light transmitter and a first magnet, such 
that the light transmitter transmits a light through an opening in the 
housing. Similarly, a remote receiver houses a light receiver and a second 
magnet. The method includes the steps of placing the remote transmitter 
against a first surface of the mounted substrate with the first magnet 
substantially resting against the first surface such that the light 
transmitter is directed towards the first surface. The remote receiver is 
located against an opposing surface of the mounted substrate with the 
second magnet substantially against the opposing surface and substantially 
in a light path alignment with the remote transmitter. The remote 
transmitter self aligns to the remote receiver according to first and 
second magnetic lines of flux of the first and second magnets, 
respectively. The light transmitter is caused to emit a light through the 
mounted substrate for detection by the light receiver and the percent 
transmittance of the mounted substrate is thereafter determined. 
According to a second embodiment of the present invention a self aligning 
window transmittance meter measures a light transmittance of a fixed 
window. The self aligning window transmittance meter includes a remote 
transmitter having a first housing for holding a first magnet in a 
predetermined alignment with a light transmitter, the remote transmitter 
also having an input for receiving a transmit signal. A remote receiver 
has a second housing for holding a second magnet in a predetermined 
alignment with a light receiver, the remote receiver also having an output 
for transmitting a transmittance signal, wherein the remote transmitter is 
placed against a surface of the fixed window such that the remote receiver 
is brought proximate to an opposing surface of the fixed window and within 
a magnetic flux of said first magnet such that the first and second 
magnets are releasably held against the surface and opposing surface, 
respectively, so as to cause said remote transmitter and remote receiver 
to self align for measuring a light transmitted through the fixed window 
by the light transmitter and received by the light receiver for providing 
a light transmittance measurement of the fixed window.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1A shows a pictorial diagram of a window transmittance meter, wherein 
the window transmittance meter (also interchangeably referred to as a base 
unit). The present invention comprises a meter 10 for measuring the 
transmittance of a substrate. The meter 10 is supported and protected by a 
housing 1 which includes a slot or receptacle 2 formed therein having a 
closed end (distal) and an open end (proximate) for sliding the housing 1 
over an edge of a substrate to be measured, for example, a window pane 4. 
The meter 10 is normally disabled (for example, a battery is electrically 
disconnected therefrom). Inserting the window pane 4 completely into, or 
nearly completely into the slot 2, causes the window pane 4 to make 
physical contact with a switch or enable circuit 5 which further causes 
the battery (not shown) to be electrically connected to the meter 10. The 
switch 5, which is located at or near the closed or distal end of slot 2, 
can be one of many types of switches currently available. For example, the 
switch 5 could be accomplished by using a normally open switch which is 
caused to be closed by the window pane 4. Similarly, the switch 5 may be 
realized by use of a momentary switch that closes only momentarily when 
contacted. Alternately, the switch 5 need not be a mechanical switch, but 
rather the switch 5 could be a laser or light operated switch, etc. 
While the switch 5 has been described advantageously as existing at the 
distal end for ensuring that the window pane 4 is properly inserted before 
making a measurement, one skilled in the art will appreciate that another 
choice of the switch 5 is a hand operated switch that powers the window 
transmittance meter for taking a measurement without further operator 
interaction. In such a case, the operator will note the change in the 
reading from that of air to that of the tinted window. This is 
accomplished by repeating the measurement every few seconds. 
After the switch 5 is contacted by the window pane 4 and battery power is 
applied to the meter 10, a light transmitter (including, for example, a 
Light Emitting Diode (LED) 9 encased in a baffle 8) begins to transmit 
light. The transmitted light travels in two distinct light paths: first, a 
portion of the transmitted light shines directly onto a first light 
detector, for example, a photodiode 12, through a tunnel or light pipe of 
the baffle 8; and second, through the inserted window pane 4 and onto a 
second light detector (including , for example, a photodiode 11 encased in 
a baffle 7). The photodiode 12, also encased in the baffle 8, but angled 
away from the LED 9, and out of the line of sight of the photodiode 11, is 
used for collecting a portion of the transmitted light for control 
purposes. The baffles 7 and 8 (having a portion cutaway for descriptive 
purposes) are provided for blocking stray, and hence undesirable light, 
while also providing a rigid housing for protecting and properly aligning 
the LED 9 and the photodiodes 11 and 12. The baffle 8 has two tunnels 
provided therein (not shown), a first tunnel for directing light through a 
substrate baffle 6, and a second tunnel for allowing light to fall upon 
the photodiode 12. The baffle 7 has a single tunnel for collecting light 
through a substrate baffle 6' and onto the photodiode 11. 
The baffles 7 and 8 may be constructed from many suitable materials, 
including, for example, urethane or ABS. The baffle 8 is preferably 
constructed of an opaque plastic, urethane or neoprene type material while 
the baffle 7 is constructed of a dark plastic, urethane or neoprene type 
material. The substrate baffles 6 and 6' are each affixed around an 
opposing surface area of the slot 2 for conforming substantially to the 
substrate or window pane 4 for blocking stray and reflected lights from 
the photodiode 11. Preferably, the substrate baffles 6 and 6' should be 
made of a material that can allow the window pane 4 to be easily inserted 
into the slot 2 while conforming to the surface of the inserted window 
pane 4. This may be accomplished for example, by a rubber or urethane type 
material having a cloth like cover wherein the cloth like cover 
sufficiently contacts the window pane 4 such that stray light is 
effectively blocked from between the interface of the window pane 4 and 
the substrate baffles 6 and 6' (i.e., the window pane 4 is sandwiched 
between the baffles 6 and 6'). 
FIG. 1B provides a side view along line A of the meter 10 in FIG. 1A. A 
portion of the substrate baffle 6 as it sits in the slot 2 is shown with a 
front side of the baffle 8 now visible. From FIG. 1B, it can now be seen, 
that with the window pane 4 inserted into the slot 2, transmitted light 
from the LED 9 passes through a relatively large opening created by the 
substrate baffle 6 (and substrate baffle 6') while stray light is 
effectively blocked. In an alternative embodiment, the substrate baffle 6 
(and 6') could be formed to provide only a very small hole, through which 
a snout (not shown) of the baffle 7 (a corresponding hole receives a snout 
of the baffle 8 on the opposing side of the slot) would be aligned. Thus, 
holes in the substrate baffles 6 and 6' and the aligned baffles 7 and 8 
allow mainly only the desired light transmitted via the LED 9 to pass 
through the window pane 4. 
The baffle 8 is shown with an oval opening or light pipe 22, which opening 
22 allows the transmitted light to pass through the baffle 8 and 
eventually onto the photodiode 11. The LED 9, substantially centered in 
the opening 9, will still transmit its light effectively to the photodiode 
11 even if the housing 1 is twisted in operation somewhat causing the 
baffle 8 to become slightly misaligned with the baffle 7. The internal 
surface of the opening 22 is smoothed for improving reflections of the 
light transmitted by the LED 9. An opening in the baffle 7 (not shown), on 
the other hand, is fitted substantially to the photodiode 11 and has an 
inside surface that is roughed or textured for reducing light reflections 
for minimizing stray light effects. The transmitted light, can also be 
generated by using a trio of LEDs, for example, a red, green and yellow 
LED, arranged in a triangle to provide a white light. 
Based upon the transmitted light being directly received by the photodiode 
11, and a portion of the transmitted light being received by the 
photodiode 12, a transmittance measurement is determined. Within a matter 
of seconds of inserting the window pane 4 into the slot 2, a transmittance 
measurement is displayed on a display 3 (FIG. 1A). The measurement may be 
held on the display 3 until the meter 10 is removed, or alternatively, a 
new measurement can be repeated every few seconds. 
FIG. 2 shows a block diagram of the preferred embodiment of the present 
invention depicting the meter 10 including the enable circuit or switch 5 
connected for providing power to circuitry making up the meter 10 when the 
window pane 4 (FIG. 1A) has been fully (or substantially) inserted into 
the slot 2. With power applied, a light transmitter circuit 200, which 
includes the LED 9, operates to provide the transmitted light to detector 
circuits 300 and 400, which include the photodiodes 11 and 12, 
respectively. The transmitted light, in the preferred embodiment, is 
produced from a green (approximately 560 nanometers) LED light source. As 
may be appreciated by one of ordinary skill in the art, a light having a 
differing wavelength can be used, for example, white light, without 
varying from the scope of the invention described herein. 
A display circuit 500 includes an oscillator and display driver 580 
(hereinafter display drive 580) and the display 3. A modulation signal, 
LCLK, generated from an oscillation signal, HCLK, of the display driver 
580 is connected to the light transmitter circuit 200 wherein the 
modulation signal oscillates at a predetermined frequency for modulating 
the LED 9 and therefore the transmitted light of the light transmitter 
circuit 200. A frequency of three kilohertz is used in the present 
invention as the frequency of the modulation signal, LCLK. The oscillation 
frequency of the display driver 580 is set to run at approximately 
forty-eight kilohertz, and is connected to the display 3 for providing a 
needed alternating current signal thereto. The three kilohertz modulation 
signal is derived from the oscillation signal, HCLK. 
Modulating the LED 9 (i.e., the transmitted light), in part, eliminates 
errors that would otherwise occur due to DC offset voltages present in the 
circuitry of the meter 10. Further accuracy is achieved by detecting only 
a band of frequencies of the transmitted light, thereby substantially 
immunizing the detector circuits 300 and 400 to low levels of stray light 
reflected off the window pane 4. The modulation signal of three kilohertz 
is at a substantially lower frequency than that of the oscillator signal 
driving the display driver 3. The modulation signal is thus divided down, 
for example, by sixteen, from the forty-eight kilohertz oscillator signal. 
As can be appreciated by those skilled in the art, the LED 9 does not 
absolutely require modulation, but instead, a tradeoff can be made for 
reducing circuit complexity at a cost of some reduction in accuracy. The 
tradeoff is dependent, to some degree, upon that stray light may be 
eliminated (creating a different complexity and cost). 
The transmitted light radiates directly upon the photodiode 11 (FIG. 1A) 
via a light path B which forms part of a light detector circuit 370. The 
light detector circuit 370 generates a detect signal proportional to the 
level of transmitted light detected, the detect signal being connected to 
a filter 380 wherein the filter 380 is a band pass filter for allowing 
only a band of predetermined frequencies to pass therethrough. The band of 
frequencies, of course, encompass the frequency of the transmitted light 
modulated at three kilohertz. In the preferred embodiment the high 
frequency is set at approximately ten kilohertz and the low frequency is 
set at approximately one kilohertz. The relatively broad band of pass 
frequencies allow the modulation frequency to vary substantially without 
significantly degrading amplitude response. If the LED 9 is not modulated 
then the filter 380 could be omitted. 
The filter 380 outputs a filtered detect signal to a peak detector 390 for 
detecting only the positive peaks of the filtered detect signal. The peak 
detected signal, VSIG, is then provided to the display circuit 500 for 
calculating and displaying the percent transmittance of the window pane 4. 
The detector circuit 400 comprises a light detector which includes the 
photodiode 12 (FIG. 1A) located just beyond the direct line of sight of 
the photodiode 9. The photodiode 12 receives the transmitted light via 
light path C, for providing a reference detect signal to a filter 480 
which filters out those frequencies below about one kilohertz. Optionally, 
a filtered indirect light detect signal of the filter 480 is connected to 
an input of a peak detector 490. The peak detector 490 detects only the 
negative peaks of the filtered indirect light detect signal. The peak 
detector 490 provides a feedback signal to the light transmitter circuit 
200 (negative feedback) for controlling an operating point of the light 
transmitter circuit 200. 
FIG. 3 shows the display circuit 500 of the meter 10 in schematic diagram 
form. A battery 701, for example a nine volt battery, is connected between 
ground and a first terminal of the switch 5. When the switch 5 is closed 
by the presence of the window pane 4, power is applied to the display 
circuit 500, and more specifically to a V+ input of the display driver 
580. The display driver 580 may be realized by an integrated circuit, for 
example, ICL71165 or TC7106TPL. The display driver 580 provides two 
regulated voltages. An analog ground voltage (COM) has a magnitude 
substantially equal to a voltage of the battery 701 less 2.8 volts. A 
digital ground voltage (DGND) has a magnitude substantially equal to the 
voltage of the battery 701 voltage less five volts. Such regulated 
voltages are then provided to power other portions of the circuitry of the 
meter 10. 
The oscillator signal, HCLK, is also supplied by the display driver 580, in 
this case the forty-eight kilohertz signal, as determined by a capacitor 
501 and a resistor 502. A divide by sixteen circuit 621, for example, 
74HC163, divides the forty-eight kilohertz oscillator signal down to the 
desired three kilohertz signal, LCLK, for modulating the transmitted light 
accordingly. Outputs of the display driver 580 are connected to the LCD 
display 3, for example, LCD1169. Reference levels and the input signal, 
VSIG, from the peak detector 390 are input into the display driver by 
devices 503 through 511. 
A magnitude of the voltage of the battery 701 is monitored to determine 
whether the magnitude is sufficient to properly operate the meter 10. If 
the magnitude of the voltage of the battery 701 falls below a 
predetermined threshold, as determined by the voltage divider connected 
resistors 601 and 602, then a comparator 603 connected thereto outputs a 
signal to an exclusive-Or gate 606. An output of the exclusive-Or gate 
606, LOWBAT, indicates a low voltage condition which can be displayed on 
the display 3. 
Referring to FIG. 4, the detector circuit 300 includes the photodiode 11 
having a cathode connected to VSS and to ground via parallel connected 
capacitors 371 and 372. An anode of the photodiode 11 is connected to an 
inverting input of the filter 380. The filter 380 includes series 
connected low pass and high pass filters realized by transimpedance 
amplifiers 377 and 382, respectively. The low pass filter is realized by 
parallel connected capacitor 375 and resistor 376 across an output and the 
inverting input of the amplifier 377. A resistor 378 connects a 
non-inverting input of the amplifier 377 to ground. RC network made of 
capacitor 379 and resistor 381 connects an output of amplifier 377 to a 
non-inverting input of the amplifier 382 (the high pass filter). Parallel 
connected capacitor 384 and resistor 383 are connected across an output of 
the amplifier 382 and an inverting input. Resistor connects the inverting 
input of the amplifier 382 to ground. Series connected capacitor 385 and 
resistor 386 connect the output of the amplifier 382 to an inverting input 
of an amplifier 391. The peak detector 390 is comprised of devices 391 
through 397 and 361 through 366. VSIG is output from the peak detector 390 
for indicating the transmittance of the window pane 4. 
The high pass filter, as previously stated is set at approximately ten 
kilohertz while the low pass filter is set at approximately one kilohertz. 
The low pass and high pass filters thus form a band pass filter (380) 
having a substantially broad frequency pass band relative to the three 
kilohertz transmitted light frequency. The locations of the poles and 
zeros may vary to reflect the wavelength of the transmitted light. Also, 
by keeping the poles and zeros at least one octave from the modulation 
frequency, frequency variation over temperature will cause insignificant 
variation in amplitude response. 
Referring to FIG. 5, the detector circuit 400 includes the photodiode 12 
for receiving the portion of the transmitted light and having a cathode 
connected to ground by capacitors 471 and 472. An anode of the photodiode 
12 is connected to an inverting input of transimpedance amplifier 483. 
Devices 481 through 486 form the low pass filter 480 in a manner similar 
to the low pass filter described above. An output of the lowpass filter 
480 is connected to an input of the negative peak detector 490 made up of 
devices 491 through 498. 
An output of the optional peak detector 490 is connected to a non-inverting 
input of amplifier 201 which acts both as a driver and a subtracter for 
controlling the LED 9. Series connected resistors 461 and 462, connected 
between ground and an inverting input of the amplifier 465 provide for 
manual calibration of an operating point of the LED 9, for example, 
approximately thirty milliamps through the LED 9. Resistors 463 and 464 
are connected for setting the gain of the amplifier 465. Resistor 205 
connects an output of the amplifier 465 to an inverting input of the 
amplifier 201. An output of the amplifier 201 drives a base of transistor 
202. An emitter of the transistor 202 is connected to the inverting input 
of the amplifier 201 via a resistor 203 and to ground via a resistor 204. 
Transistor 202 can thus be biased to control a current flow through the 
LED 9. The LED 9 is modulated by the signal LCLK which is connected to the 
base of a transistor 211 by a resistor 212. A collector of the transistor 
211 is connected to an anode of the LED 9 while a cathode of the LED 9 is 
connected to a collector of the transistor 202. A filter circuit made up 
of devices 206 through 209 connect the emitter of the transistor 211 to 
ground, COM, and VSS. Additionally, parallel connected capacitors 631 
through 636 (FIG. 3) provide a filter between VSS and ground. 
Referring now to FIG. 6, a circuit board 21, for holding circuit components 
of the meter 10 is shown as mounted within the housing 1 (see FIGS. 1A and 
1B). The circuit board 21 includes an upper portion 33, and a wide leg 31 
and a narrow leg 32 situated on opposing sides of the slot 2. Three 
standoffs, 23, 24 and 25, fasten the circuit board 21 to the housing 1. 
The wide leg 31 is fastened by standoff 24 while the upper portion 33 of 
the circuit board 21 is fastened by standoffs 23 and 25. The narrow leg 32 
is not fastened, but rather is allowed to float. By using three standoffs 
23, 24 and 25 while allowing the narrow leg 32 to float, the circuit board 
is adequately attached to the housing 1 while providing flexibility such 
that a twisting motion applied to the housing 1 will not cause fractures 
or other failures in the circuit board 21. This method of attachment 
allows the housing 1 to twist or stress while the circuit board 21 remains 
more rigid. Additionally, this allows the LED 9 (FIG. 1A) to remain better 
aligned to the photodiode 11 during such stress. Alternatively, the narrow 
leg 32 could be attached such that a hole holding a standoff has adequate 
free-play to allow the narrow leg freedom to remain unaffected by some 
twisting of the housing 1. The standoffs 23, 24 and 25, may be made, for 
example, from rubber, urethane, or other suitable materials. Still 
further, if a light diffuser is used over the LED 9, thus allowing for 
additional misalignment between the LED 9 and the photodiode 11, then the 
narrow leg 32 may be attached in a standard fashion as is the wide leg 31. 
The foregoing described substrate transmittance meter 10 meets the 
requirements of being portable, accurate and reliable. However, this meter 
is unable to measure substrates or windows that do not have an accessible 
edge (a fixed window is defined as a window or substrate that has all 
edges surrounded, for example, a rear window that is non-movable, such 
that no edge is exposed for measurement purposes). The usefulness of the 
substrate transmittance meter 10 is improved and extended to further 
measure transmittances of fixed windows by the invention of the inventors 
herein. A problem of accurately measuring the transmittances of fixed 
windows is properly aligning a receiver and transmitter on opposing sides 
of the fixed window (the transmitter and receiver can no longer be mounted 
in a predetermined fixed relationship). The inventors herein provide a 
novel solution including providing a remote transmitter and a remote 
receiver, each having a donut shaped magnet that cause the remote 
transmitter and remote receiver to self align on opposing sides of the 
fixed window for making an accurate transmittance measurement. 
FIG. 7A depicts a remote transmitter 700 having a housing 701 with a 
suction cup 709 hingedly attached thereto by a hinge mechanism 711. The 
hinge mechanism 711 further includes a press fit snap that firmly holds 
the suction cup 709 against the housing 701 for storage purposes. Held 
within the housing 701 are a baffle 703 and light transmitter 713 which 
are similar in materials and purpose to the baffle 8 and light transmitter 
9 as shown in FIG. 1A. The baffle 703 and light transmitter 713 are held 
in place and aimed through an opening in the housing 701 (coming out of 
the page). A light diffuser 715 (FIG. 7B) is provided covering the light 
transmitter 713 for diffusing the light thus allowing for some 
misalignment between the remote transmitter 700 and remote receiver 800 
(approximately 10 millimeters). 
One or more donut shaped magnets 705 (see FIG. 7B) are held in place at a 
bottom of the housing 701--the light transmitter is pointed through holes 
of the donut shaped magnets 705. The donut shaped magnets 705 are glued or 
otherwise permanently joined to form a cylinder type structure wherein 
magnetic flux lines of each magnet 705 are additive, i.e., the several 
magnets 705 join to form a single stronger magnet. A felt-like material 
721 is attached to an outermost edge of the magnets 705 for interfacing 
with the substrate or window (providing both scratch protection and stray 
light blocking). Dual optical centering lines 707 are provided on the 
housing 701 for providing a visual indication of the internal location 
(optical center) of the light transmitter 713. 
In operation, the light transmitter 713 (for example, an LED) receives a 
transmit signal for transmitting a light. If the substrate transmittance 
meter 10 is used as a base unit, the LED 9 is disabled and the light 
transmitter 713 receives the transmit signal (the signal that would have 
powered the LED 9, including LCLK). Alternatively, the light transmitter 
circuit 200, the light detector circuit 300 and display 500 may be 
integral to the remote transmitter 700 and/or remote receiver 800. 
FIG. 8A depicts the remote receiver 800 having a housing 801. While there 
is not a suction cup attached to the housing 801, such a suction cup may 
be included on the housing 801 instead of or in addition to the housing 
701 (FIG. 7) such that the remote receiver is releasably attached to the 
fixed window instead of the remote transmitter. Held within the housing 
801 are a baffle 803 and light receiver 813 (i.e., a photodiode) which are 
similar in materials and purpose to the baffle 7 and light receiver 11 as 
shown in FIG. 1A. The baffle 803 and light receiver 813 are held in place 
for receiving a transmitted light through an opening in the housing 801 
(going into the page). 
One or more donut shaped magnets 805 are held in place (FIG. 8B) at a 
bottom the housing 801--the light receiver 813 is pointed through a center 
of the holes of the donut shaped magnets 805. The donut shaped magnets 805 
are glued or otherwise permanently joined to form a cylinder type 
structure wherein magnetic flux lines of each magnet 805 are additive, 
i.e., the several magnets 805 join to form a single stronger magnet (north 
poles connected to south poles). A felt-like material 821 (or other 
suitably soft material) is attached to an outermost edge of the donut 
shaped magnets 805 for interfacing with the substrate or window. Dual 
optical centering lines 807 are provided on the housing 801 for providing 
a visual indication of the internal location (optical center) of the light 
receiver 813 and for rotational alignment with the light transmitter 700. 
In operation, the light receiver 813 receives the transmitted light from 
the remote transmitter 713 through the fixed window. If the substrate 
transmittance meter 10 is used as a base unit, the photodiode 11 is 
disabled and the light receiver 813 transmits a transmittance signal 
(VSIG, the signal that would have otherwise driven the display 500) to the 
base unit. Alternatively, the light transmitter circuit 200, the light 
detector circuit 300 and display 500 may be integral to the remote 
transmitter 700 and/or remote receiver 800. 
METHOD OF OPERATION 
FIG. 9 shows the remote transmitter 700 being releasably attached to an 
outside surface of a fixed window. The suction cup 707 released from a 
storage position and swung away from the housing 701 about a hinge. The 
suction cup 709 is then attached to the outside surface of the fixed 
window such that optical centering lines are located between window 
defrost lines 901 if they are present. With the suction cup 709 attached 
to the fixed window the remote transmitter 700 rests against the outside 
surface of the fixed window wherein its own weight provides sufficient 
force to form a substantially light blocking interface between the 
felt-like material 921 and the fixed window. 
Communication ports 903 accept wires for receiving the transmit signal from 
the base unit (if used) and for further receiving VSIG or its equivalent, 
the signal generated by the light receiver 813 as a result of measuring 
the transmitted light, and transmitting VSIG back to the base unit (if 
used). 
FIG. 10 shows the relationship between the remote transmitter 700 and the 
remote receiver 800. The remote transmitter 700 is temporarily held in 
place and need not be moved again. The donut shaped magnets 805 of the 
remote receiver 800 is then brought near the inside or opposing surface of 
the fixed window near the donut shaped magnets 705 of the remote 
transmitter 700. The force of the magnetic flux lines of the donut shaped 
magnets 705 will attract the magnetic flux lines of the donut shaped 
magnets 805 and bring the remote receiver 800 into optical alignment with 
the remote transmitter (not rotational alignment but and edge-to-edge 
alignment). If there are defrost lines 901 in the fixed window the remote 
receiver 800 should be rotated so that the optical center ribs align on 
both units. If the fixed window does not have defrost lines 901, then the 
relationship of the optical center ribs is of no consequence. 
FIG. 11 shows the transmittance measurement being performed with the remote 
transmitter 700 and the remote receiver 800 in place on the fixed window 
and coupled to the base unit 10. When the remote receiver 800 was brought 
into place near the remote transmitter 700, it was not necessary to 
attempt to exactly align the two remote units. The respective donut shaped 
magnets 705 and 805 self align the two units within a number of 
millimeters. A communication port 1101 on the base unit 10 causes the 
internal LED 9 and photodiode 11 to become disabled. The external light 
transmitter 713 and light receiver 813 receive the appropriate signals in 
place thereof. A transmittance measurement is enabled by pressing an 
enable switch 1103 on the base unit 10. The enable switch 1103 operates in 
place of the switch 5 of FIG. 1A. The resulting transmittance is then 
displayed on the base unit 10. While the remote transmitter 700 has been 
shown first attached on the outside surface of the fixed window, the 
remote receiver 800 could first be releasably attached to the outside 
surface of the fixed window with the remote transmitter 700 on the inside 
surface. The inside surface of the fixed window is the preferred location 
for locating the remote receiver 800 since the tinted fixed window would 
help block some stray light from reaching the remote receiver 800. 
Although the present invention has been described in relation to particular 
embodiments thereof, many other variations and modifications and other 
uses will become apparent to those skilled in the art. For example, the 
materials chosen may vary so long as the physical and/or electrical 
requirements are met. The communication ports 903 and 1101 and the wires 
907 can be replaced by other viable means of communication, for example, 
infrared or radio transmitted signals. The base unit 10 may not be 
necessary in the case where all of the electronics (less the LED 9 and 
photodiode 11) are integrated into the remote transmitter 700 and remote 
receiver 800 in some form. Furthermore, transmittance or opacity 
measurements are not meant to be limited to tinted glass or fixed 
automobile windows. Therefore, the present invention is limited only by 
the claims.