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Luminescent tags are molecular highlighters. They enable light-based biological assays, visualize biological processes, and are used as diagnostic and therapeutic tools in biomedicine. Since the molecules of interest are usually of low concentration, the light emission is faint, on the order of femtowatt to nanowatt level of power, but accessible to photodetectors and sensitive cameras currently available.


“What is the lowest amount of light that this instrument can detect?” is an important yet unmet measurement challenge. Evaluating the detection performance of complicated imaging instruments is crucial to quantifying substances of interest and the ability to compare results across instruments, methods, and laboratories. This can be met by using SI-traceable units and employing radiometric methods to compare optical detection systems.

This project is about finding the appropriate tools to calibrate instruments, especially imaging ones, in order to evaluate detector performance. Calibration tools may be detectors or sources which carry SI-traceable light units and whose scale can be transferred to working standards. Working standards are often user community standards such as fluorescent glass slides, glass beads or tissue phantoms. Without the absolute scale, working standards especially for fluorescence, remain only relative intensity standards as they are dependent on excitation irradiance that may also not be easily quantifiable in an instrument.

We can calibrate dim sources on the order of femtowatt to nanowatt total power and report as intensity or radiance. Calibration needs are application-specific; contact us for consultation.


Maritoni Litorja
Yuqin Zong
C. Cameron Miller


Zhu, B., Sevick, E. and Litorja, M. (2019), Comparison of NIR versus SWIR fluorescence imaging of indocyanine green using SI-derived metrics of image performance, IEEE Transactions on Medical Imaging, [online],

Pogue, B., Zhu, T.C., Nitziachristos, V., Paulsen, K.D., Wilson. B.C., Pfefer, J., Nordstrom, R.J., Litorja, M., Wabnitz, H., Chen, Y., Gioux, S., Tromberg, B.J., Yodh, A.G., “Fluorescence-guided surgery and intervention-An AAPM emerging technology blue paper” Medical Physics Vol. 45, 6, June 2018, 2681-2688.

Litorja, M. , Lorenzo, S. , Zhu, B. and Sevick, E. (2016), Lambertian nature of tissue phantoms for use as calibrators in near infrared fluorescence imaging, Molecular-Guided Surgery: Molecules, Devices, and Applications II (Conference 9696), San Francisco, CA, [online],

Zhu, B., Litorja, M., Sevick, E. and Rasmussen, J. (2016), Determining the Performance of Fluorescence Molecular Imaging Devices Using Traceable Working Standards with SI Units of Radiance, IEEE Transactions on Medical Imaging, [online], 

Created May 12, 2021, Updated May 13, 2021