A Novel Scanning Lens Instrument for Evaluating 

Fresnel Lens Performance: 

Equipment Development and Initial Results

Rebeca Herrero1, 2, David C. Miller2, Sarah R. Kurtz2, Ignacio Antón1 and Gabriel Sala1 

1 Instituto de Energía Solar-Universidad Politécnica de Madrid, Spain

2 National Renovable Energy Laboratory, Golden, CO, USA

CPV-9: International Conference on Concentrator Photovoltaic Systems 

April 15-17, 2013, Miyazaki, Japan

NREL/PR-5200-59215
Outline

• Optical Performance Characterization: 

• Definition 

• Light Source and Flux Measurement Device

• Lens Scanning Instrument Development

• Lens Scanning Instrument vs. CPV Solar Simulator

• Results 

• Conclusions 
Optical Performance Characterization: Definition

( ) ( )

( λθφ )

λθφ λθφ

,,,,

,,,,

,,,,

ininininin

outoutoutoutout

ininininray yxP

yxP yxT =

Transmittance Function

Optical Efficiency

Integrating over:

yx ,,,, λθφ

• The optical efficiency is dependent on angular, spatial and spectral features of the light source

• The optical efficiency is dependent on the receiver used:

• Optical coupling

• Spectral and angular responsivity

• Sensor size

 How can we measure the optical efficiency?
Light Source and Flux Measurement Device

• Light Source

– Spectrum

– Collimated Light

– Examples: 

• Sun 

• Xenon Flash Lamps + Optics

• Lasers

• Flux Measurement 

Device

• Thermal Sensors 

 (e.g, Pyranometer) 

• Multi-Junction Solar Cells

(or isotype cells)

• CCD Si Sensor

Equipment requirements for lens 

performance & aging evaluation:

• Ability to quantify and diagnose local defects

 (vs. measure the overall lens efficiency)

• Ability to examine at different wavelengths

• Large sensor size

Scanning Lens Instrument (SLI) 

at NREL
Scanning Lens Instrument Development

( ) ( )

( ) λ

λ λη

,,

,,

,,

ininin

outoutout

inin yxP

yxP yx =

85C/85%RH 

(1000h.)

SunTest 

( 1000 h. UV)

Scratched SoG 

PMMA

INTENTIONAL 

DAMAGE

CCD image of laser light

CCD image of laser arriving to 

the sensor through the lens Integrating over (x,y) 

Scanning 

lens 

aperture

Light spot for λ 

Map of optical efficiency for λ, 

η Average optical efficiency for λ 

Reference

+ 

SLI at NREL 

Lasers (405nm, 632nm, 650nm) 

Lens

MEASURE

 η 
Scanning Lens Instrument Development: Lasers

• Spatial and Angular Distribution of Lasers (405nm, 532nm, 650nm)

• Laser partially coherent 

• Refractive random surface diffuser to generate multiple light sources 

(reduce coherence)

Average Profile Obtained from CCD Images

Blue (405nm) Green (632nm) Red(650nm) Blue (405nm) Green (632nm) Red(650nm)

SPATIAL ANGULAR 
Scanning Lens Instrument vs. CPV Solar Simulator

• Lens characterization: transmittance and optical efficiencies

Scanning Lens Instrument (SLI) at NREL

( ) ( )

( ) λ

λ λη

,,

,,

,,

ininin

outoutout

inin yxP

yxP yx = ( ( ) ( ))

inin

outout

in in AG

AG LE ⋅⋅ ,, θφλη =

CPV Solar Simulator at IES-UPM 

RELATIVE DIFFERENCES COMPARED WITH REFERENCE SAMPLE ARE STUDIED

Multi-junction

Solar Cell

Is the measured optical efficiencies at SLI in correspondence with 

the other characterizations?

η Blue (405nm), η Green (532nm) , η Red (650nm) η Top, η Middle 

Lasers (405nm, 632nm, 650nm) 
Scanned Lens Area

Results: Localized Defect Quantification

mm

Map of optical transmission 

efficiency (Resolution 0.2mm for 

green laser 530nm)

Scratch in the silicone

Scratch in the glass surface

Optical image of a 

scratched lens 

(front and back surfaces)

Image detail of the same 

scratch in the silicone 

95% to 94%

Lasers (405nm, 632nm, 650nm) 
UV UV

SunTest SunTest

Lenses PMMA at SLI

Transmittance vs. wavelength, PMMA Transmittance vs. wavelength, PMMA

Lenses PMMA at CPV solar simulator

Results I: Lenses PMMA in SLI & Other Characterizations 

Lens Lens
Transmittance vs. wavelength, SoG Transmittance vs. wavelength, SoG

Lenses SoG at SLI Lenses SoG at CPV solar simulator

Results II: Lenses SoG in SLI & Other Characterizations 

Lens Lens
Future Work

• Repeat SLI and Solar Simulator characterization with more 

lenses and aging conditions 
Conclusions

• The SLI equipment does not provide an absolute optical 

efficiency value, but the effects of lens aging can be 

quantified and diagnosed

• Component characterization and image processing has been 

required to validate the SLI results 

• Both PMMA and SoG lenses have been measured in the SLI 

and Helios CPV solar simulator, with similar results to 

spectrophotometer transmittance
Thank you for your attention