If you are using Chrome 25+ and have a webcam in addition to your spectrometer, Google has hidden the "camera selection" menu under Settings > Advanced Settings > Privacy > Content Settings > Media > Camera. This is really unfortunate and we hope Google will improve that interface in the future.
For Chrome 21-24, you can just choose a camera in the dropdown to the upper-right corner, then click Allow to the upper left.Rotate image 90°
(Beta): Your spectrometer samples just one row of pixels, so adjust this slider until the yellow line intersects the brightest, clearest part of your spectrum. You'll have to point this at a light bulb or bright light source.
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