AMMT Optical Path and Performance 

The optical system for laser 

injection and in-line process 

monitoring within the system 

control module is shown on 

the top right diagram. 

Imager is aligned co-axially 

with the laser to capture 

‘stationary’ images of the melt 

pool.

Objective: Optimize imager 

performance at 850 nm

Constraints: Attain <150 mm 

FWHM laser spot at 1070 nm

Variables: LTZ, CLP, and CIL 

lens curvature, glass type, 

distance

Result: MTF 64 cycle/mm

Schematic of ray 

tracing model 

used to optimize 

components for 

1:1 imaging in 

process monitoring 

beam path.

Data for l = 825 nm to 875 nm

Build point shown is 

(x,y) = (30, 30) mm

X

Y

(x,y) = 

(30,30) mm 40 mm

Modeled 

Build Plane

Airy disk contour is for 850 nm

Radius = 1.22λ(f/#) 

(x,y) = 

(30,24) mm Relative Attenuation

Left to Right: Coordinates in optical model, spot diagram for modeled imager performance, and 

modeled imager MTF estimating 64 cycle/mm resolution.

TEMPS Optical Path and Performance

• Reflective mirrors with protected silver coating to 

avoid chromatic aberrations

• diffraction limited performance on-axis over the 

full wavelength range (.38 μm to 10 μm)

• Near-diffraction limited performance over a 2 mm 

diameter field of view.

Three flat BaF2 plates:

- nearest to the scanner acts as a beamsplitter

with a front surface reflecting the fiber laser;

- The 2

nd provides a gas seal and partial 

compensation for the chromatic aberration 

caused by the beamsplitter plate

- The 3

rd provides the remainder of the 

chromatic aberration compensation.

System Optimization

Implementation Features

Temperature & 

E

M

P

S

mittance of 

elts,

owders and

olids

Emissivity Realization Options

• Scanning Laser Emissometer with Inline Broadband Metrology

(DyME Method)

• Novel Center-Mount Reflectometer (folded virtual sphere) VIS

• Constant Optical Path Trajectory (maintain imaging quality

across the spectrum)

• Separate Sample Tray/Heater for large laser beam and long

interaction mode (sub-seconds and longer)

Dynamic Meltpool Emissometry (DyME) Hybrid Method: New Realization

Additive Manufacturing and other applications, involving directed energy beams to selectively melt or

sinter materials, can benefit from better understanding of emissive properties of materials in solid,

powder and liquid states, as well as improved techniques for real time temperature determination.

To address these priority needs, AMMT Testbed incorporates TEMPS, a system for characterizing the

optical and radiative properties of metallic, polymer and ceramic materials in solid, powder and molten

states at temperatures up to 3300 K at static as well as dynamic conditions of the heated zone.

Earlier Facility Limitations Measurement Capabilities

and New Opportunities

Anticipated Impact

and Beneficiaries

• Limited T Range (resistive heating)

-Use Laser Heating 

• Sample Containment 

-Use local heating (variation of a “Cold Skull”) 

• Long Dwell Time at High T (sample degradation) 

-Use Scanning Laser Beam + Fast Spectrometer

• Only Solid Samples (no melts or powders)

-Use center-mount virtual sphere reflectometer

• Limited T Range for Reference Sources

-Use Solid State / Laser Sources

Promote Innovation via better Non-Contact 

Thermometry, Radiative Transfer Analysis 

and Process/Target Engagement Control in:

• Laser-Based Additive Manufacturing and 

Material Processing Technologies

• Refractory and Composite Materials for 

aerospace, nuclear reactors and 

thermionic systems

• Directed Energy Systems and their 

counter-measures

1A- 1B Measure reflectance/emittance at single wavelength with filter radiometer

2A – 2B Measure radiance at same wavelength with filter radiometer, Calculate T

3A-3B Measure spectral radiance with spectrometer, Calculate spectral emittance 

• Very few model assumptions

• Polymers, ceramics and metals

• Powders, solids and melts

• Very high T’s possible

• Wide spectral coverage

• T uniformity across FoV

• Triggering and integration 

• Imaging is very demanding

• Plume effects

• Highly reflective materials

Key TEMPS Components:

Reflectometer, Standards and Radiometric Modules

• Treats moving melt pool or hot spot as a static target; reduces

contamination & evaporation due to the small heat affected zone

• Reflectance and emittance measurements utilize a shared optical

path with heating and probing lasers.

• Path integration leads to several seconds of total measurement

time and enables good signal-to-noise ratio

Directional-Hemispherical Reflectance (DHR):

 Probe laser light incident on sample at near-normal

 Reflected light is sampled by radiometers viewing VIS 

wall

 S/N Advantage: sample emitted light has same 

throughput as probe laser reflected light

Virtual Integrating Sphere

Reflectometer (VIS)

Future Studies for Uncertainty Evaluation

• Imaging artifacts due to out-of-field scatter

• Temperature distribution across the FoV

• Plume, surface layer ablation, plasma absorption effects

• Thermodynamic equilibrium conditions

Radiometric Module and Light Path Standards Module and

Reflectometer

Gas Blanket and Plume 

Evaluation Setup

HYBRID METHOD (Pierluigi Testa, Rome University, 1975):

- INDIRECTLY measure EMITTANCE (reflectance) at a SINGLE λ*

(a) Measure RADIANCE T at λ*

(b) Calculate TRUE SURFACE TEMPERATURE at λ*

- DIRECTLY Measure EMITTANCE across the spectral range

Removable 

Splitter/Reflector 

(RSR)

Custom Imaging Lenses 

(CIL)

Fold 

mirror

Laser 

reflector

Custom 

LTZ

Window

(fused silica)

Custom relay lenses

(may or may not be used)

12 mm circular 

source object on 

build plane

Scanner and Fold 

Mirrors

(Note: only one of three 

mirrors shown)

Custom 

Converging Lens Pair 

(CLP)

Image

Image

To vacuum window and 

TEMPS toroidal mirror 

(when RSR is replaced with 

laser-line reflector) 

Laser

200 mm

BaF2substrate

beamsplitter

Toroidal Mirror:

100 AOI

Fold mirror 

BaF2 Window/ 

compensation plate

External BaF2

compensation 

plate

Field stop location

Scanner

500 mm

Scanner Mirrors and Build Plane Toroidal Mirror

TEMPS Module 

FT Spectrometer 

ADVANTAGES LIMITATIONS 

Si Array 

Spectrometer 

Gas Blanket - Return