Abstract: A stimulus values direct reading colorimetric instrument comprising a plurality of photoelectric sensors having different spectral responsivities equal or approximately equal to color matching functions X(.lambda.), Y(.lambda.), and Z(.lambda.); and measuring and display device to measure and display the tri-stimulus values X,Y and Z and the chromaticity of the light to be measured by computing the photoelectric sensor output signals caused by the light, wherein the instrument calculates the stimulus values corresponding to the tri-stimulus values and measures the chomaticity by providing at least one corrective photoelectric sensors so as to maximize their spectral responsivities in the vicinity of the transmission threshold wavelengths of each sensor and taking the sum of the output signals from the sensors and the corrective sensor after multiplying each coefficient assigned to the signals by each output signal, respectively.

Abstract: An input protecting mechanism for a measuring device, such as a digital multi-meter, wherein a shutter is provided to open and close selectively input terminal holes for leads, depending on the particular measuring function selected so that the test lead cannot be placed in an improper terminal, so that advantageously, damage to the internal circuitry is prevented by preventing erroneous signals from being applied to the wrong circuits.

Abstract: An integrating analog-to-digital converter comprising an integrator, a memory capacitor for memorizing the potential of an integrated output from the integrator, a comparator for comparing the potential memorized in the memory capacitor and the potential of the integrated output divided by a set of potential dividing resistances, a clock pulse generating circuit, a counter for counting the period until the integrated output of the reference voltages passes through a reference level using clock pulses, and a reversible counter for counting the period from when the integrated output passes through the reference level to when the integration is completed, by the clock pulses, and for adding its value to the counted value from the counter as low order digits; wherein the number of digits of AD conversion is increased, and high conversion accuracy and high resolution are realized by enlarging the time period from the reference level crossing point to a clock pulse immediately after the crossing in the reference vo