What should be the first course of action if a spectrophotometer's readout does not rise above 90%T with the light path fully open?

When a spectrophotometer's readout does not rise above 90% transmittance (T) with the light path fully open, the most logical first course of action is to replace the source lamp. This is because the lamp is essential for providing adequate light intensity for measurements. If the lamp is failing or weak, it may not emit sufficient light, resulting in lower transmittance readings regardless of any other settings or configurations.

Thus, if the light output is compromised due to a faulty lamp, even with an unobstructed light path, correct readings cannot be achieved. Therefore, addressing the lamp issue is paramount to ensuring proper instrument functionality and reliability in measurements.