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Modern high-efficiency engines utilize direct injection for high-pressure charge preparation. At these conditions atomization occurs on the micrometer and nanosecond scales. While high-speed cameras can shutter at tens of nanoseconds and long-range microscopes can be coupled to these cameras, these system's resolution is typically limited by pixel size and field of view. The large pixels inhibit diffraction-limited optical resolution and limited data throughput under high repetition rates reduces the sensor area. A novel technique is needed to image fuel atomization at engine-relevant conditions. In this work, we demonstrate a new high-resolution imaging technique, Spectral Microscopy, which realizes diffraction-limited imaging at framerates sufficient for capturing primary breakup in engine-relevant sprays. This system utilizes a commercial DSLR, allowing for wider FOV and improved resolving power compared to high-speed cameras. Temporal shuttering is accomplished via three independently triggered illumination sources. The RGB image sensor captures independent red, green, and blue light pulses, thereby capturing three consecutive images at effective framerates exceeding 20 Million fps. We present the system design, including a high-throughput backlit illumination system, a high magnification a multi-piece lens, and an image processing algorithm to formulate a three-frame temporally-resolved image. The system is then demonstrated by imaging engine relevant diesel sprays. The Spectral Microscopy system in this work resolves micron-scale features at framerates exceeding 20 Million fps, expanding the capability for experimental research on fuel spray primary breakup in modern direct-injection engines.