Shawn C. Taylor

The University of Toledo, Toledo, Ohio

Noncontacting Laser Inspection System for

Dimensional Profi ling of Space Application

Thermal Barriers

NASA/CR—2011-216803

March 2011
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Shawn C. Taylor

The University of Toledo, Toledo, Ohio

Noncontacting Laser Inspection System for

Dimensional Profi ling of Space Application

Thermal Barriers

NASA/CR—2011-216803

March 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contract NNC08CA35C

Prepared for the

Fourth Japan US Symposium

cosponsored by the Japanese Society for Non-Destructive Inspection (JSNDI) and the American Society 

for Non-Destructive Testing (ASNT)

Maui, Hawaii, June 7–11, 2010
Acknowledgments

The author would like to thank Irebert Delgado, NASA Glenn Research Center (GRC), Jeffrey DeMange, The University of 

Toledo, and John Mayer, Analex Corporation for their participation in the statistical evaluation of the laser inspection system. 

This work was supported by the National Aeronautics and Space Administration under contract NNC08CA35C.

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Level of Review: This material has been technically reviewed by NASA expert reviewer(s).
NASA/CR—2011-216803 1 

Noncontacting Laser Inspection System for Dimensional Profiling of 

Space Application Thermal Barriers 

Shawn C. Taylor 

The University of Toledo 

Toledo, Ohio 43606 

Abstract 

A noncontacting, two-dimensional (2-D) laser inspection system has been designed and implemented 

to dimensionally profile thermal barriers being developed for space vehicle applications. In a vehicle asinstalled state, thermal barriers are commonly compressed between load sensitive thermal protection 

system (TPS) panels to prevent hot gas ingestion through the panel interface during flight. Loads required 

to compress the thermal barriers are functions of their construction, as well as their dimensional 

characteristics relative to the gaps in which they are installed. Excessive loads during a mission could 

damage surrounding TPS panels and have catastrophic consequences. As such, accurate dimensional 

profiling of thermal barriers prior to use is important. Due to the compliant nature of the thermal barriers, 

traditional contact measurement techniques (e.g., calipers and micrometers) are subjective and introduce 

significant error and variability into collected dimensional data. Implementation of a laser inspection 

system significantly enhanced the method by which thermal barriers are dimensionally profiled, and 

improved the accuracy and repeatability of collected data. A statistical design of experiments study 

comparing laser inspection and manual caliper measurement techniques verified these findings. 

Introduction 

The continuous push for the evolution of space flight technology creates an unceasing need for new 

and improved vehicle components and systems that will enable future missions to significantly exceed the 

realm of current possibilities. Thermal barriers (TB) are used on space vehicles to fill gaps between 

external thermal protection system (TPS) panels (e.g., tiles and ablators) and protect lower temperature 

vehicle components and structures (actuators, etc.) from the harsh, high temperature environment created 

during atmospheric reentry. Thermal barrier failure can have catastrophic effects which could ultimately 

lead to loss of vehicle and crew. 

Significant research is being conducted in cooperation with NASA Glenn Research Center (GRC) to 

develop and evaluate cutting edge TB concepts to satisfy the stringent requirements for current and future 

space vehicles. A typical TB requirement is the ability to maintain sufficient resilience, in oxidizing 

environments with gas temperatures sometimes exceeding 3000 °F, such that contact is maintained 

against dynamic, load sensitive, mating TPS panel surfaces. Thermal barriers are commonly fabricated 

from a combination of high temperature ceramic fabrics and superalloy resilient elements. These barriers 

are typically installed under a nominal level of compression to allow them to track both expansions and 

contractions of the gaps in which they are installed. An example of an installed TB is shown in Figure 1. 

Resultant loads exerted by a TB when it is compressed between two TPS panels are directly related to the 

TB’s original height. In the orientation depicted in Figure 1, the height of the installed TB is the 

horizontal dimension, not the vertical dimension. If loads are excessive, adjacent TPS panels can be 

damaged. As such, TB geometry must be accurately characterized so that tolerable compression ranges 

can be incorporated into vehicle designs. Due to the compliant ‘soft good’ nature of thermal barrier 

construction, measurement of height and width using conventional contacting techniques, such as calipers 

and micrometers (Fig. 2), is subjective. Researchers inspecting TB samples are required to make a 

judgment as to when contact between the TB surface and the measurement device is achieved. Obtaining 

repeatable TB measurements using these methods is difficult because the thermal barriers deform with 

even the slightest contact, and variance in data collected by different researchers can be large. 
NASA/CR—2011-216803 2 

Figure 1.—Representative configuration 

of a thermal barrier installed between 

two TPS panels. 

Figure 2.—Manual techniques 

classically used for thermal 

barrier measurement: a.) dial 

caliper, and b.) depth micrometer 

with precision parallels. 

To improve the TB measurement process, a noncontacting, two-dimensional laser inspection system 

was assembled from commercially available components and implemented in a research setting. The 

goals of implementing the laser system were to streamline the characterization of thermal barrier test 

sample geometries, and improve the overall accuracy and repeatability of the data collected. Once the 

system was operational, a statistical analysis using a formal design of experiments (DOE) approach was 

conducted to quantify the effectiveness of the laser inspection system, and verify that the goals of its 

implementation were achieved (Ref. 1). For this study, a three-factor, full-factorial statistical design was 

selected to examine the effect of three primary factors (1) measurement method, (2) researcher, and (3) 

TB sample, on two response variables (1) measured mean height, and (2) measured mean width, of a 

candidate TB design. 
NASA/CR—2011-216803 3 

Description of the 2-D Laser Inspection System 

The primary components of the two-dimensional laser inspection system, configured for analyzing 

linear thermal barrier segments, are highlighted in Figure 3. The Keyence LJ-G200 laser head used in this 

study has an x-axis range of 2.87 in. (±0.00078 in. repeatability) and a z-axis range of ±1.89 in. 

(±0.0038 in. linearity, ±0.00008 in. repeatability). The laser head is mounted to an Etalon vertical stage 

that facilitates precision adjustment of the laser head to accommodate thermal barriers of varying height. 

The vertical stage is mechanically attached to an optical base alongside a Zaber actuated horizontal stage. 

In this configuration, the laser head is positioned above the horizontal stage with the laser beam directed 

toward an inspection platform (attached to the stage) upon which TB samples are placed for evaluation. 

The laser’s x-axis of measurement is orthogonal to the direction of stage travel, and the z-axis is in the 

vertical direction (Fig. 4). Using the actuated horizontal stage, TB samples can be moved relative to the 

stationary laser without physical contact from the operator, allowing accurate dimensional data to be 

collected from multiple locations along the length of the sample. The horizontal stage has an 11.8 in. 

range of travel across which actuation can be automated using associated computer software. The laser 

head is connected to a Keyence controller (model LJ-G5001) which displays profile readings on an LCD 

monitor. The controller also has the ability to interface with an external PC using associated software for 

automated data collection. 

Screen images of the LCD monitor during a representative TB inspection are shown in Figure 5. 

Figure 3.—2-D laser inspection system for 

thermal barrier dimensional profiling. 

Figure 4.—Photograph of a thermal 

barrier on the laser inspection 

platform highlighting measurement axes, the horizontal stage 

direction of travel, and the laser 

beam. 
NASA/CR—2011-216803 4 

Figure 5.—Screen images of the LCD 

monitor during a representative 

thermal barrier inspection: a.) 

measurement display mode, and b.) 

profile display mode. All values were 

intentionally zeroed. 

Laser Effectiveness Study 

Procedure 

Ten thermal barrier samples, approximately 5 in. in length, were selected from the same fabrication 

lot of a candidate TB design under consideration at NASA GRC. These samples were assigned unique 

identifications to facilitate tracking throughout the study. Four different researchers collected three sets of 

height and width measurements from each of the 10 TB samples, first with a digital caliper (Mitutoyo 

500-785, ±0.001 in. accuracy), and then with the newly integrated 2-D laser inspection system. The more 

subjective caliper measurements were collected first to prevent potential bias in the results. The laser 

system was operated in a fully manual capacity, i.e., data was read from the LCD monitor and recorded 

into a notebook by hand, and motion of the horizontal stage was controlled without use of the PC 

interface software. Collected data was evaluated in Minitab using a statistical analysis of variance 

(ANOVA) with a level of significance, , of 0.05. Due to the difference in magnitude of the height and 

width measurements, the two data sets were evaluated independently. 

Results and Discussion 

Preliminary inspection of TB samples using the 2-D laser system showed many benefits over contact 

measurement methods classically employed in TB research. The noncontacting nature of laser inspection 

minimized the necessity of researcher judgment in data collection and made TB measurement more 

objective. Unlike classical methods, laser inspection did not influence TB measurements. TB dimensional 

data collected using calipers was often biased smaller than actual values because caliper contact deformed 
NASA/CR—2011-216803 5 

the compliant TB surface enough to skew inspection results. During laser measurement, however, a TB 

was placed on the inspection platform, and was not physically touched until the inspection was complete. 

This minimized the introduction of contact bias into the sensitive measurements. In addition, the 2-D 

capability of the laser allowed width and height measurements to be collected simultaneously. Previously, 

two separate physical measurements were required. TB characterization using the 2-D laser inspection 

system was quicker and could be automated. Combined, the many benefits of laser inspection are 

expected to make TB geometric characterization more efficient, accurate, and repeatable. 

Thermal Barrier Height Analysis 

Examination of the ANOVA results revealed that the main factors of researcher and TB sample had 

statistically significant effects on the measurement of TB height at a 0.05 level of significance. The effect 

of researcher is highlighted on the effects plot shown in Figure 6. A two-factor interaction between 

measurement method and researcher also had a statistically significant effect on measured height. 

Although measurement method was not determined to have a statistically significant influence on 

measured TB height as a main factor, the effects plot presented in Figure 6 does show a trend toward 

smaller mean height when calipers were used. This was expected, as contact between the caliper and the 

TB surface has been shown to bias height measurements toward erroneously small values. 

Practically interpreted, results from the TB height analysis showed that the most influential factors 

affecting the characterization of TB mean height were the TB samples themselves, and the researcher 

collecting the data. Despite satisfying specified manufacturing tolerances, height variation due to the 

compliant nature of the TB samples was still large enough to be statistically significant at a significance 

level of = 0.05. The influence of researcher on data collection was also expected to be significant, given 

the level of subjectivity associated with TB height measurement with calipers. Although the effect of 

measurement method itself was not determined to be significant, there was significance detected due to 

measurement method-researcher influence interaction. The difference between the maximum and 

minimum mean height measured by all four researchers, using calipers, was 0.031 in. That difference was 

only 0.011 in. when the values were collected with the laser inspection system, a 65 percent decrease. 

This proved that implementation of the 2-D laser inspection system did enhance the precision with which 

TB mean height was characterized. 

Thermal Barrier Width Analysis 

Analysis of variance of the collected width data, like the analysis of the TB height data, indicated that 

the main effect, TB sample, and a two-factor interaction effect between measurement method and 

researcher, were statistically significant at a 0.05 level of significance. There was not sufficient statistical 

evidence, however, to suggest that the main effect of researcher alone significantly influenced the 

measurement of mean TB width. This difference between the results of the height and width analyses is 

attributed to an observation that the TB samples used in the study were noticeably less compliant in the 

width direction than they were in the height direction. Increased rigidity in the width direction is believed 

to have reduced the level of subjectivity in the measurements, thereby minimizing variability in the 

collected data. The main effects plot presented in Figure 7 shows that the magnitudes of the effects of 

measurement method and researcher on recorded TB mean width were within the range of accuracy of the 

measurement techniques employed. 

The benefits of laser implementation were less pronounced in the analysis of the TB width data; 

however, researchers did note that the overall process of data collection was undeniably more efficient 

when the 2-D laser inspection system was used in place of the classical caliper measurement method. 
NASA/CR—2011-216803 6 

Figure 6.—Main effects plot highlighting the influence of both 

measurement method and researcher on thermal barrier mean 

height measurements. Y-axis labels are presented as the difference 

of each reported height from the mean of all recorded heights. 

Figure 7.—Main effects plot showing the minimal influence of 

measurement method and researcher on thermal barrier mean 

width measurements. Y-axis labels are presented as the difference 

of each reported width from the mean of all recorded widths. 
NASA/CR—2011-216803 7 

Conclusions 

A noncontacting, two-dimensional, laser inspection system was implemented in a research setting at 

NASA GRC to characterize the geometry of candidate thermal barrier designs for space vehicle 

applications. Observations made during evaluation of the laser inspection system led to the following 

conclusions: 

1. The 2-D laser system improves upon classical contact measurement methods by eliminating the 

need for physical contact with TB samples during measurement. This minimizes the need for 

researcher judgment during TB inspection, thereby making the inspection process more objective, 

and collected data more accurate and repeatable. By collecting both height and width 

measurements simultaneously, laser measurement makes the TB inspection process more 

efficient. 

2. A statistical analysis using a three-factor, full-factorial design of experiments verified the 

advantage of laser TB inspection over traditional contact-based inspection techniques. The 

analysis identified researcher, TB sample, and an interaction effect between researcher and 

measurement type as significant factors that influence TB measurement, as expected. Although 

the effect of geometric variation between TB samples cannot be mitigated by changing inspection 

techniques, the deleterious influence of the other significant factors can be minimized through 

laser implementation. The enhanced precision that the laser system provides was highlighted by 

the fact that the range of mean height values collected using the laser inspection system during 

this study was 65 percent smaller than the range of the data collected using a caliper. 

3. The 2-D laser inspection system significantly enhances the process by which thermal barriers are 

characterized in a research setting. The system could be easily adapted for incorporation into 

future applications such as design qualification testing and pre-flight acceptance where the 

benefits of noncontacting inspection could prove to be invaluable. Use of the laser system in preflight operations would minimize the chance for TB damage by reducing the handling necessary 

for inspection, and could enable technicians to find TB flaws that may not have been detected by 

traditional inspection techniques. These benefits would ultimately lead to enhanced vehicle safety 

and reduced risk for future flight crews and critical space hardware. 

References 

1. Montgomery, Douglas C. Design and Analysis of Experiments. New York: John Wiley and Sons, 

Inc., 2001. 
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Noncontacting Laser Inspection System for Dimensional Profiling of Space Application 

Thermal Barriers 

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14. ABSTRACT

A noncontacting, two-dimensional (2-D) laser inspection system has been designed and implemented to dimensionally profile thermal 

barriers being developed for space vehicle applications. In a vehicle as-installed state, thermal barriers are commonly compressed between 

load sensitive thermal protection system (TPS) panels to prevent hot gas ingestion through the panel interface during flight. Loads required 

to compress the thermal barriers are functions of their construction, as well as their dimensional characteristics relative to the gaps in which 

they are installed. Excessive loads during a mission could damage surrounding TPS panels and have catastrophic consequences. As such, 

accurate dimensional profiling of thermal barriers prior to use is important. Due to the compliant nature of the thermal barriers, traditional 

contact measurement techniques (e.g., calipers and micrometers) are subjective and introduce significant error and variability into collected 

dimensional data. Implementation of a laser inspection system significantly enhanced the method by which thermal barriers are 

dimensionally profiled, and improved the accuracy and repeatability of collected data. A statistical design of experiments study comparing 

laser inspection and manual caliper measurement techniques verified these findings.

15. SUBJECT TERMS

Experiment design; Thermal protection; Dimensional measurement; Lasers; Heat shielding; Ablative materials 

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