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This report is available at no cost from the National Renewable Energy

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Contract No. DE-AC36-08GO28308

Conference Paper

NREL/CP-5D00-76896

September 2020

Using Spectral Measurements to 

Characterize Solar Reference Cells on a 

Two-Axis Tracker

Preprint

Frank Vignola,1 Josh Peterson,1 Rich Kessler,1

Aron Habte,

2 Peter Gotseff,2 Manajit Sengupta,2 and

Fotis Mavromatakis3

1 University of Oregon

2 National Renewable Energy Laboratory

3 Hellenic Mediterranean University 

Presented at the 47th IEEE Photovoltaic Specialists Conference (PVSC-47)

June 15–August 21, 2020
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Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

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Conference Paper

NREL/CP-5D00-76896

September 2020

Using Spectral Measurements to 

Characterize Solar Reference Cells on a 

Two-Axis Tracker

Preprint

Frank Vignola,1 Josh Peterson,1 Rich Kessler,1

Aron Habte,2 Peter Gotseff,2 Manajit Sengupta,2 and 

Fotis Mavromatakis3

1 University of Oregon 

2 National Renewable Energy Laboratory

3 Hellenic Mediterranean University 

Suggested Citation 

Vignola, Frank, Josh Peterson, Rich Kessler, Aron Habte, Peter Gotseff, Manajit 

Sengupta, and Fotis Mavromatakis. 2020. Using Spectral Measurements to Characterize 

Solar Reference Cells on a Two-Axis Tracker: Preprint. Golden, CO: National Renewable 

Energy Laboratory. NREL/CP-5D00-76896. https://www.nrel.gov/docs/fy20osti/76896.pdf.

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Using Spectral Measurements to Characterize

Solar Reference Cells on a Two-Axis Tracker

Frank Vignola

University of Oregon

Solar Radiation Monitoring Lab

Eugene, Oregon, USA

fev@uoregon.edu

Josh Peterson

University of Oregon

Solar Radiation Monitoring Lab 

Eugene, Oregon, USA 

jpeters4@uoregon.edu

Rich Kessler

University of Oregon

Solar Radiation Monitoring Lab 

Eugene, Oregon, USA

rkessler@uoregon.edu

Aron Habte

National Renewable Energy 

Laboratory

Golden, Colorado USA

Aron.Habte@nrel.gov

Peter Gotseff

National Renewable Energy Laboratory

Golden, Colorado USA

Peter.Gotseff@nrel.gov

Manajit Sengupta

National Renewable Energy Laboratory

Golden, Colorado USA

Manajit.Sengupta@nrel.gov

Fotis Mavromatakis

Dept. of Electrical & Computer Engineering

Hellenic Mediterranean University

Estavromenos, Crete, Greece

fotis@hmu.gr

Abstract – A study is commencing relating the spectral 

responsivity of reference cells in the field to changing spectral 

distribution during the day. Initial results indicate that 

measurements of the incident spectral distribution can be used to 

match results of reference cell measurements on a two-axis tracker 

with a standard deviation of ±0.25% under clear-sky conditions. 

Initial results are provided. As more data are obtained covering a 

variety of field conditions and locations, the robustness of these 

results will be tested. 

Keywords – reference cell, spectral, tracker, photodiode-based 

pyranometer

I.INTRODUCTION

The National Renewable Energy Laboratory (NREL) and 

the University of Oregon Solar Radiation Monitoring 

Laboratory (UO SRML) are jointly evaluating the performance 

of several reference cells on two-axis trackers using spectral 

measurements and Class A pyranometers. The data became 

available from NREL on May 1, 2020 and a similar experiment 

is being installed in Eugene, Oregon in June, 2020. This is part 

of a project to understand and characterize the measurements 

from reference cells and photodiode-based pyranometers to 

facilitate the comparisons of field experiments made using a 

variety of instruments.

The availability of ground-based irradiance measurements 

is limited compared to the coverage of satellite-derived 

irradiance data, however, ground-based measurements can 

provide measurements at shorter timescales and ones that are 

more specific for a given photovoltaic (PV) site. Ground-based 

data can also be made available in real time to assist station 

operation.

Reference cells and photodiode-based pyranometers have 

been studied previously [1]–[4] and have shown systematic 

biases when compared to high-quality thermopile-based 

pyranometers. The output of reference cells and photodiodebased pyranometers are dependent on the spectral distribution 

of incident radiation, the temperature of the sensors, and the 

angle-of-incident (AOI) effects, dependences that are similar to 

those found in PV modules. Knowledge of these dependencies 

can help when estimating the incident irradiance from reference 

cell measurements.

This study also aims to examine the performance of a 

variety of reference cells on two-axis trackers at the high 

altitude facility in Golden, Colorado and in the verdant 

Willamette Valley in Eugene, Oregon. Also mounted on the 

trackers are spectroradiometers and Class A pyranometers (see 

Fig. 1). The two-axis tracker provides a platform with the 

instruments pointed directly at the sun, hence minimizing the 

AOI effects. (There are some AOI effects from diffuse and 

ground reflected irradiance especially when the sun is within 

10o of the horizon.)

Fig. 1 Two-axis tracker and instruments at NREL. A one-axis tracker with 

similar instruments is in the background.
2

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

II.ABOUT THE DATA

In Golden, Colorado the platform includes FRA, ATO, IKS, 

NES, IMT, and EET reference cells as well as a LI-COR LI200R and a Kipp & Zonen SP Lite2 pyranometer. At Eugene, 

Oregon the platform includes RC18, IKS, NES, IMT, and EET 

reference cells as well as a LI-COR LI-200R and a Kipp & 

Zonen SP Lite2 pyranometer. By testing the output of the 

reference cells against spectral distributions for two diverse 

sites, site-specific or equipment specific effects can be 

estimated. The data from these experiments are uploaded to

NREL’s Measurement and Instrumentation Data Center as they 

are gathered.

Reference solar cells are used to evaluate the performance 

of photovoltaic (PV) systems. The advantage of using 

references cells is that they are similar in structure and material 

to PV modules being evaluated; therefore, reference cells will 

have the same spectral response and AOI response as PV 

modules. The disadvantages are that they are not pyranometers 

designed to measure incident solar radiation, and their spectral 

response and AOI response differ from Class A pyranometers. 

Models used to estimate solar irradiance on different surfaces 

and orientations are based on high-quality measurements made 

using Class A pyranometers and pyrheliometers. This makes it 

difficult to use reference cell measurements for comparisons or 

validation with models or measurements made to using 

irradiance data obtained from Class A instruments.

NREL and UO SRML have been studying the performance 

of reference cells and pyranometers on horizontal, fixed, and 

one-axis tracking surfaces [1-4]. Based on these studies, it was 

decided to instigate two-axis tracking systems both Golden and 

Eugene to evaluate the performance of a variety of reference 

cells and to obtain a more general understanding of the 

reference cells and the uncertainties involved.

III.DATA

The data comes from several reference cells mounted on a 

two-axis tracker along with a Class A Kipp & Zone CMP 22 

pyranometer and EKO spectroradiometers with a combined 

range from 290 to 1650 nm. In Eugene, the EKO 

spectroradiometer has a range from 300 to 1100 nm. The CMP 

22 pyranometer acts as a reference pyranometer. A comparison 

of an IMT reference cell to the reference pyranometer is shown 

in Fig. 2 for two clear-sky periods, the morning of May 3, 2020,

and the afternoon of May 4, 2020.

In the late afternoon and early morning (solar zenith angle 

= 80o to 85o), the temperatures are about 25 C and the 

differences are related to different spectral distributions and 

different ground reflected and horizon irradiance.

The relationship between IMT output and irradiance is 

given in Eq. 1.

IMT = K⸱R(λ,T) F(AOI) (1)

where R(λ,T) is the responsivity of the reference cell summed 

over all wavelengths (λ). The effect of temperature (T) on the 

reference cell responsivity is also wavelength dependent and is 

included in the model of R(λ,T) [3], [4]. The constant K scales 

the spectral responsivity of the reference cell. F(AOI) describes 

the deviation from a cosine response as a function of the AOI). 

For normal incidence irradiance on a two-axis tracker F(AOI) = 

1.

Fig. 3 shows the ratio of R(λ,T) calculated using the spectral 

distribution measured by the spectroradiometers to the 

measured output of the IMT reference cell during clear-sky 

periods. The reference cell and spectroradiometer are both 

mounted on the two-axis tracker.

The calculation of average responsivity also involved a 

model of the spectral temperature dependence of the reference 

cell [2]–[4]. The average temperature response of a reference 

is usually the only parameter that is measured. In the case of the 

IMT reference cell, the average temperature dependence is 

proportional to 0.0008 times the difference between the 

reference cell temperature and 25oC.

A comparison using the spectral verses fixed temperature 

dependence is shown in Fig. 4. When the reference cell is near 

25 C, the two calculations agree. For this example this occurs 

early in the morning and late in the afternoon when the solar 

Fig. 3 Ratio of calculated responsivity of the IMT to IMT output on a 

two-axis tracker under clear-sky conditions.

Fig. 2 Plot of the ratio of the global normal irradiance from a CMP 22 

pyranometer and an IMT reference cell during clear-sky periods on May 3, 

2020 and May 4, 2020. Instruments mounted on a two-axis tracker.

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

20° 25° 30° 35° 40° 45° 50° 55° 60° 65° 70° 75° 80° 85° 90°

Global Normal Ratio of IMT to

CMP 22 measurements

Solar Zenith Angle (Degrees)

Global Normal Ratio of IMT/CMP22

May 3 and 4, 2020 - Golden, CO
3

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

zenith angle is between 80° and 85°. Using the spectral 

temperature dependence reduces the standard deviation by 

approximately 40% as compared to a fixed temperature 

dependence.

IV. CONCLUSIONS

Using Eq. 1 and information in Fig. 3, the value of K 

determined from 20° to 80° is 0.5872 with a standard deviation 

of± 0.0015 or approximately ±0.25% for the clear-sky period 

during these 2 days. This illustrates that the average 

responsivity of a monocrystalline reference cell can be modeled 

accurately up to 80° if the spectral distribution is known and the 

instrument is on a two-axis tracker (point directly to the sun).

Assuming ground reflected irradiance has only a small 

contribution, this method can be adopted to other surfaces and 

orientations if one has a solid model of F(AOI) given 

information on incident spectral irradiance. More usefully, one 

should be able to extract the average responsivity of the 

reference cell and use that information to estimate PV system 

performance.

Although spectral irradiance data are seldom available, 

models for spectral irradiance do exist, and with the data 

provided by these experiments, spectral models can be tested; 

therefore, it should be possible to covert reference cell 

measurements into irradiance values.

ACKNOWLEDGMENTS

The UO Solar Radiation Monitoring Laboratory would like to 

thank the National Renewable Energy Laboratory as well as the 

Bonneville Power Administration, Energy Trust of Oregon, and 

Portland General Electric for the support that makes this work 

possible. This work was authored in part by Alliance for 

Sustainable Energy, LLC, the manager and operator of the 

National Renewable Energy Laboratory for the U.S. 

Department of Energy (DOE) under Contract No. DE-AC36-

08GO28308.via the U.S. Department of Energy under Contract 

No. DE-AC36-08GO28308. Funding provided by U.S. 

Department of Energy Office of Energy Efficiency and 

Renewable Energy Solar Energy Technologies Office. The 

views expressed in the article do not necessarily represent the 

views of the DOE or the U.S. Government. The U.S. 

Government retains and the publisher, by accepting the article 

for publication, acknowledges that the U.S. Government retains 

a nonexclusive, paid-up, irrevocable, worldwide license to 

publish or reproduce the published form of this work, or allow 

others to do so, for U.S. Government purposes.

REFERENCES

[1] F. Vignola, C. Chiu, J. Peterson, M. Dooraghi, M. 

Sengupta, “Comparison and analysis of instruments 

measuring plane-of array irradiance for one-axis tracking 

PV systems”, IEEE PVSC Conference, Washington D.C., 

2017

[2] F. Vignola, J. Peterson, M. Dooraghi, M. Sengupta, F. 

Mavromatakis, “Comparison of Pyranometers and 

Reference Cells on Fixed and One-Axis Tracking” 

American Solar Energy Society Conference Denver, 

Colorado October 9–12, 2017

[3] Frank Vignola, Josh Peterson, Rich Kessler, Mike 

Dooraghi, Manajit Sengupta, and Fotis Mavromatakis, 

“Evaluation of Photodiode-based Pyranometers and 

Reference Solar Cells on a Two-Axis Tracking System”, 

World Conference on Photovoltaic Energy Conversion

Waikoloa, Hawaii, June 10-15, 2018

[4] Frank E Vignola, Josh Peterson, Rich Kessler, Vikram 

Sandhu, Aron Habte, Manajit Sengupta. “Improved Field 

Evaluation of Reference Cell Using Spectral 

Measurements”, IEEE PVSC Conference, Chicago, IL,

2019

Fig. 4 Ratio of calculated responsivity of the IMT to IMT output on a 

two-axis tracker under clear-sky conditions using fixed and spectral 

temperature dependences.

0.55

0.57

0.59

0.61

0.63

0.65

20° 25° 30° 35° 40° 45° 50° 55° 60° 65° 70° 75° 80° 85° 90°

Ratio of Calculated to Measured IMT Values

Solar Zenith Angle (Degrees)

Comparison of Calculated and Measured IMT Output 

for Fixed and Spectral Temperature Dependence

Clear Skies - May 3-4, 2020 - Golden, CO

Fixed Temp Dependence Spectral Temp Dependence