transmission values: what to look for… in order to look

Y.M.WP.Transmission Values By Joe DeMonte 7.11.12 v1

Transmission Values: What to Look For… in Order to Look Through!

By Joe DeMonte

ASNT/PdM Thermal Infrared Level III

Purpose: The purpose of this paper is to educate on the subjects of:

1. Transmission values for infrared windows: Does a high value equal more accuracy?

Or, is knowing the percentage of transmission more important?

2. Transmission values for infrared windows: How does a “grill” effect transmission?

Will it make quantitative thermography null and void?

Tested Components: One electrical lug and one hot plate heated to steady state conditions. One

polymer infrared window with a mesh style safety guard and one crystal calcium fluoride (CaFl)

infrared window. One generic plastic garbage bag. One stainless steel cup filled with water and

heated to steady state conditions.

Tools Used During the Test: FLIR E50 thermal imager, thermometer

Definitions:

Transmission, transmissivity: The ability of light in the specified wavelength to pass through an

object. Example: visible light through a pane of glass, LW infrared light through a polymer infrared

window.

Thermography: is a technology that creates and analyzes images by detecting the heat radiating from

an object.

Quantitative Thermography: Analyzing thermal images by acquiring temperature values.

Qualitative Thermography: Analyzing thermal images without acquiring temperature values.

Steady State Conditions (Heat Transfer): When heat transfer is occurring while the temperature of

the objects involved do not change.

LW/Long Wave/8-12um: The waveband of most modern infrared maintenance cameras. The metric

unit micrometer/micron/um is used to measure the wavelength.

Non-Gray/Selective Radiator: Most substances and materials will not keep the same value of

transmission, reflection, and emission throughout the light spectrum. This may result in different

thermography cameras obtaining different results when measuring the same material.


Introduction:

As an active thermographer and instructor for Infrared courses, I’ve always considered

infrared windows as an exciting technology. In the 90’s, there weren’t many choices when talking

about IR windows. Nowadays, we have several. I will cover two major materials used in modern

infrared windows: polymers and fluoride crystals. Both have value for the particular band width

(referred to as LW/Long Wave/8-12um) of infrared cameras that 99% of the thermographers use

during condition based monitoring.

Recently, I have been made aware of a rumor that polymer windows affixed with a mesh style

safety guard cannot be used for accurate quantifiable temperature measurements. I had not

considered this as an issue during my role as a thermographer or heard of it as an issue from the

100’s of electricians that ran our infrared program for TEGG Corporation around the globe. I decided

that I had better double check our methods and see for myself if I had been doing things incorrectly

for years.

Test Parameters

In order to measure transmission through a material, I will propose two procedures. We have

to remember that the results of the tests may not apply to other cameras even with the same

wavelength filters. This is primarily due to inconsistencies of detector response curves across the

waveband being used and the non-gray “selective radiator” effects of most materials in that

waveband.

Our transmissivity procedures:

• How to Measure Transmittance with a radiometric (temperature measuring) thermal imager:

1. Set your thermal imager up to measure “apparent temperatures” with e = 1.00 and

distance (if you have that setting) to 0. Also, external window optics should be off or

100%.

2. Heat up a metal target with a high emissivity target (electrical tape will work) affixed to

it to a steady state condition. I would recommend at least 30°C above reflected

temperatures.

3. Measure and set reflected apparent temperature.

4. Measure target without the IR window. Note the temperature.

5. Measure target with window in place.


6. Adjust emissivity to set the camera temperature equal to the original temp without

the window. This is NOT the emissivity measurement of your window. We are using

the camera’s emissivity as a “loss” calculation and calculating the losses through the

window. The emissivity value you arrive at to reach the original directly viewed target

temperature equates to the transmission of the IR window.

OR

1. For cameras that have EXTERNAL OPTICS TRANSMISSION, this procedure is a little bit

more accurate. Heat up a metal target with a known emissivity target affixed to it.

About 30°C or more above reflected temperature is desirable.

2. Measure the actual temperature of the target. Example: with electrical tape, set e =

0.95 and measure and set Reflected Apparent Temperature. Note the temperature.

3. Measure target with window in place.

4. Adjust external window optics temperature to the temperature of the window (the

window is best kept at room temperature).

5. Adjust external window optics transmission until you reach the actual temperature of

the target as measured without the window in place. The value of transmissivity that

you arrive at to achieve the original temperature is the transmissivity of the IR

window.


Question 1: Does a high value equal more accuracy? Or, is knowing the percentage of transmission

more important?

I will compare the results of a household garbage bag to a industrial strength polymer infrared

window and a crystal window. The polymer window should have a lower transmission value due to

the thickness/type of polymer and the reinforcing safety grill. The garbage bag should be relatively

high. The crystal will be lower than the garbage bag.

Here are the results:

Item Name Directly Viewed Parameters

Electric lug

Directly

viewed.

A r1:max 87.8

18.4

86.7 °C

50

Max Temperature 87.8 °C

External Optics Transmission 1.00

External Optics Temperature 21.0 °C

Emissivity 0.95

Electric lug

Viewed

through

polymer

window

A r1:max 55.5

21.1

55.5 °C

40




Emissivity 0.95

Electric lug

Adjusted for

transmission

A r1:max 87.5

21.2

87.4 °C

50


Emissivity 0.95



From the tests above, the 3” round polymer infrared window with a safety grid has a transmissivity of

0.46. Applying that result to an entirely different target and checking that result to a contact

thermometer, we find the following:


SS Cup

Viewed

Directly A r1:max 70.4

18.9

76.3 °C

50

Emissivity 0.95



Ar1 Max. Temperature 70.4 °C

SS Cup

Viewed

through IR

polymer

window

A r1:max 70.0

22.4

75.3 °C

40

60

Emissivity 0.95



Ar1 Max. Temperature 70.0 °C

Visual

picture of

the

thermometer

A physical reading of approximately 157°F,

which converts to: 69.44°C. Well within

the accuracy of a radiometric thermal

imager.

My initial findings were that it appears we can use the known transmission value, even though it is

only 46%, and continue to perform quantitative thermography. Will this be the transmissivity value

that all infrared cameras measure through this style IR window? Absolutely not! Perhaps this was

the fact that led to some of the confusion in the industry.


On to the plastic garbage bag….

Test Object

Directly

Viewed. A r1:max 90.8

19.4

90.6 °C

40

60

80

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

21.0

°C

Ar1 Max. Temperature 90.8

°C

Test Object

Viewed

Through a

Single Ply

Garbage

Bag

A r1:max 81.1

20.1

80.7 °C

40

60

80

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C


°C

Test Object

Viewed

Through a

Single Ply

Garbage

Bag with

Trans.

Adjusted.

A r1:max 90.9

19.9

90.7 °C

40

60

80

Emissivity 0.95

External Optics

Transmission

0.83

Max Temperature 90.9

°C

Min Temperature 19.8

°C

The test on the single ply garbage bag with the 90°C target shows a transmission measurement of

0.83 or 83%. We will change out the target to a metal cup with heated water and check the results

with our same thermometer for accuracy.


Stainless

Steel cup

with hot

water

viewed

directly.

A r1:max 69.6

19.6

69.5 °C

40

60

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C

Ar1 Max.

Temperature

69.6

°C

Object

viewed

through a

single ply

garbage

bag.

A r1:max 62.2

20.4

62.0 °C

30

40

50

60

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C

Ar1 Max.

Temperature

62.2

°C

Object

viewed

through a

single ply

garbage bag

with

transmission

adjusted.

A r1:max 69.3

20.3

69.1 °C

40

60

Emissivity 0.95

External Optics

Transmission

0.83

External Optics

Temperature

21.0

°C

Ar1 Max.

Temperature

69.3

°C

It appears that the original value of 0.83 transmission for the garbage bag holds after looking at a

different temperature object. It is a high value of transmission, but doesn’t give us any more

accuracy when performing thermography over the lower value (0.46) of the polymer/mesh window.

In other words, know the value of your window with regards to your thermal imager and don’t

worry about how high the value is.


I will now test the crystal window. It has a viewing diameter of 68mm with a thickness of 2mm.

Using the same two heated targets, here are the results:

Test object

directly

viewed. A r1:max 90.8

19.4

90.6 °C

40

60

80

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C


°C

Object

viewed

through

2mm thick

crystal

window.

A r1:max 55.4

20.7

55.2 °C

40

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C


°C

Object

viewed

through

2mm thick

crystal

window

with trans

adjusted.

A r1:max 91.1

20.4

90.7 °C

40

60

80

Emissivity 0.95

External Optics

Transmission

0.43

External Optics

Temperature

21.0

°C


°C

The crystal window has a transmission percentage of 0.43 or 43%. A little lower than the polymer

stacked with the mesh on this temperature target. I will now look at the cooler target, the hot water

in the stainless steel cup with electrical tape affixed to it.


Stainless steel

cup with

electrical tape

viewed directly.

A r1:max 69.6

19.6

69.5 °C

40

60

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C


°C

Stainless steel

cup viewed

through a 2mm

thick crystal

window.

A r1:max 45.2

21.2

45.1 °C

30

40

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5

°C


°C

Stainless steel

cup viewed

through a 2mm

thick crystal

window with

trans adjusted.

A r1:max 69.7

21.6

69.5 °C

40

60

Emissivity 0.95

External Optics

Transmission

0.45

External Optics

Temperature

21.0

°C


°C

At a transmission of 0.45 or 45%, we do obtain a slightly different value of transmission in order to

match the directly viewed result.

Fluoride crystals are very “non-gray” to the majority of thermal infrared cameras used by

thermographers (a concept that has been documented in many papers). This means that the

temperature of the viewed object will cause the perceived transmission through the crystal to change


when using long wave infrared cameras. When we are trying to measure the temperature of an

unknown object, such as found inside electrical cabinets, we cannot know the exact transmission

value to use in the camera. Because the actual value of transmission, not how high the value is,

controls our quantitative thermographic accuracy, we normally cannot achieve accurate

measurements or accurate differentials between two objects through that type of material.

I will show one more example of a high temperature target and the polymer and crystal to further the

experiment on transmissivity values.

Hot plate

viewed

directly. A r1:max 148.7

22.3

148.2 °C

50

100

Emissivity 0.95

External Optics

Transmission

1.00

External Optics

Temperature

20.5 °C


°C

Hot plate

viewed

through the

polymer/mesh

window.

A r1:max 149.0

25.8

147.8 °C

50

100

Emissivity 0.95

External Optics

Transmission

0.46

External Optics

Temperature

21.0 °C


°C

Hot plate

viewed

through the

2mm crystal

window.

A r1:max 156.3

23.8

155.6 °C

50

100

150

Emissivity 0.95

External Optics

Transmission

0.43

External Optics

Temperature

21.0 °C


°C


If I dial in the original 46% transmission value for the polymer/mesh window, I get the correct

temperature (within the IR camera manufacturer specification). The crystal is a bit off and will

continue to be further away from the actual temperature as the object’s temperature changes.

Question Number 2: How does a “grill/mesh” affect transmissivity? Can I get accurate results

through an IR window that has a safety “grill/mesh” installed?

So far, our experiment has used a polymer window with a grill/mesh. When the infrared camera is

place up close to the mesh, the camera can focus beyond the mesh and was able to obtain

consistency. I personally use the area max measurement function when viewing targets through IR

windows and directly viewed targets to keep consistency. Some instances dictate that I use a single

point temperature spot, and I prefer infrared cameras that can do both for flexibility.

To perform this experiment, let’s start by considering the grill. In my experience looking through

infrared windows installed in electrical cabinets, I find that the best/widest field of view is when the

infrared camera is close to the window. The mesh, when installed, will be out of focus and not look

like a mesh on the image. It will affect transmissivity values by lowering them, but again, we are not

so much concerned with a lower value. Even when using a non-mesh window such as a crystal,

normal operation will have the infrared camera closer to the window for a wider field of view into the

cabinet (see the following visual example). There is no need to have different procedures between

the two types of windows (mesh or non-mesh) based on this consideration.

Panel is 20” from the front of the IR/digital

camera. Crystal IR window is placed up against

the IR/digital camera. Overall FOV of the panel

is relatively wide.

Panel is 20” from the front of the IR/digital

camera. Crystal window is 6” from the front of

the IR/digital camera. Overall FOV of the panel

is down by at least ½.


I also want to document the effects of viewing angle on transmission when regarding mesh and non-

mesh windows.

My simple setup contains an electrical component heated on a hot plate for steady state purposes.

The IR polymer window with the safety grill/mesh is installed into the side of a box. It is

approximately 8” away from the electrical component (outside the minimum safe distance of 4” for

5kv cabinets).

Digital image of test setup.


Perpendicular to the target. Off angle about 30°. As far as possible (>30°)…

note that the perceived

Emittance of the target may

have also changed.

Perpendicular to the target. Off angle about 30°


The same concept using a crystal window without a safety mesh/grill:

90° to the target. Rotated about 30° to the

target.

Rotated > 30° to the target.

Emittance may have been

affected due to the angle.

It appears that we have near identical effects from both the polymer with mesh/grill and crystal

windows. It may be the foreshortening of the angle to the grill for the polymer window, and it may

be due to the increased thickness of the crystal being viewed through for the crystal window.

Another possibility is that the overall Emittance of the target will change with angle and for the

electrical tape, it will lower as you approach 45° and beyond towards parallel with the surface.


Summary: In the field, a thermographer is always going to move the camera slightly to obtain the

best picture. I would always recommend this regardless of whether you are looking through a

transmissive material or directly at the target. Consider your infrared window materials carefully and

check your sources. I have personally heard seasoned thermographers make statements that a grill

or mesh will not allow for accurate temperatures. Or that the higher the transmission, the better the

window. In our study, we have determined that both of the tested material windows have similar

transmission values for our specific infrared camera (2011 FLIR E50). The crystal window will vary the

transmission as the temperature of the object changes.

If a thermographer decides to back the camera away from the infrared window in his/her surveys, the

mesh may not affect the transmission that the camera “sees” through the window. The transmission

values will be much higher (again, knowing the value is important, not how high the value is), but are

still measureable as long as the openings of the mesh are not smaller than the overall

IFOVmeasureable (also known as Spot Size Ratio) of your specific camera. I would not recommend

this only because of the much smaller overall field of view you will have during your inspections

versus being close to the infrared window.

transmission values: what to look for… in order to look

Documents