is desirable in order to warn an operator of impending bobbin thread exhaustion which might interfere with the appearance of a long seam a number of these arrangements utilize a light source and a light detector arranged so that when there is thread on the bobbin the optical path from the light source to the light detector is blocked this path being opened when the amount of thread remaining on the bobbin is depleted below some threshold value upon the occurrence of this latter condition appropriate circuitry activates an alarm or indicator that warns the operator that the amount of thread remaining on the bobbin is below the predetermined threshold many of these arrangements have frequently been of limited reliability due to the effect of stray light producing erroneous triggering of the low bobbin thread warning alarm most attempts to eliminate the effect of stray light from such sources as room lighting and sewing machine mounted work guiding lights have been limited to enclosing the loop taker cavity in which the light sensitive photo detector resides with an opaque bed slide and spraying the cavity with a flat black paint treatment to reduce internal reflections from reaching the light detector other attempts have involved defining a narrow optical path from the light source to the light detector and providing shielding about the lift detector to block all light other than that which emanates from the light source while these prior arrangements are.