Bioluminescence imaging (BLI) demonstrates cellular events as a light signal at the single-cell level using a highly sensitive, cooled CCD camera. However, BLI signals are relative values and thus, images taken on different days or using different equipment cannot be compared directly. We established a reference LED light source that was characteristic of the total flux and light distribution and calibrated the BLI system as an absolute light signal. This calibrated BLI system revealed that the average light signal of beetle luciferase was at an attowatt level per sec at the single cell level.

Planar-type Lambertian light-emitting diodes (LEDs) with a circular aperture of several tens of μm to a few mm in diameter were developed for use as radiant-flux standard light sources, which have been in strong demand for applications such as quantitative or absolute intensity measurements of weak luminescence from solid-state materials and devices. Via pulse-width modulation, time-averaged emission intensity of the LED devices was controlled linearly to cover a wide dynamic range of about nine orders of magnitude, from 10 μW down to 10 fW. The developed planar LED devices were applied as the radiant-flux standards to quantitative measurements and analyses of photoluminescence (PL) intensity and PL quantum efficiency of a GaAs quantum-well sample. The results demonstrated the utility and applicability of the LED standards in quantitative luminescence-intensity measurements in Lambertian-type low radiant-flux level sources.

Previous epidemiological studies have revealed that occupational exposure to extremely low frequency weak magnetic fields (ELF‐WMF) is associated with the development of neurodegenerative diseases including Alzheimer’s disease (AD) and amyotrophic lateral sclerosis (ALS). The molecular mechanism of these detrimental effects, however, remain mostly unelucidated. We examined the effects of extremely low frequency fluctuation of weak magnetic fields (ELF‐WMF) on cultured mammalian cells. While applying ELF‐WMF, we examined the temporal profiles of expression levels of VDAC1, a mitochondrial outer membrane protein, and seven electron transport complex (ETC) proteins of NDUFS1 (ETC I), NDUFB8 (ETC I), SDHB (ETC II), UQCRFS1 (ETC III), UQCRC2 (ETC III), MTCOI (ETC IV), and ATP5A (ETC V). ELF‐WMF downregulated the amounts of these proteins at 3 h and recovered them at 12 h. Quantification of the amount of mitochondria with MitoTracker Green showed that the mitochondrial mass was decreased to 70% at 3 h and recovered at 12 h. In addition, mitophagy‐related proteins of PINK1 and LC3‐II were increased by ELF‐WMF at 1.5–2.5 h. Quantification of ETC enzymatic activities in protein homogenates revealed that ELF‐WMF exclusively suppressed the activity of ETC II in 10 min. We report that ELF‐WMF much weaker than geomagnetic fields suppresses the enzymatic activity of ETC II, and provokes mitophagy. Mitophagy is a cellular quality‐assurance system to eliminate damaged mitochondria. Mitophagy induced by ELF‐WMF may be causally associated with the development and progression of AD and ALS. Contrarily, ELF‐WMF finely tuned to preferentially eliminate damaged mitochondria may be able to ameliorate Parkinson’s disease, in which compromised mitophagy is a cause of the disease. Support or Funding Information This research was supported by grants‐in‐aids from MEXT, MHLW, and AMED of Japan.