Via elongational flow opto-rheometry (EFOR), simultaneous measurements of tensile stress sigma(t) and birefringence Delta n(t) were conducted on a low density polyethylene (LDPE) melt and its blends with an ultra-high molecular weight polyethylene (UHMWPE) at 140 degrees C under transient elongational flow with constant tensile strain rate (epsilon) over dot(0). The transient elongational viscosity eta(E)(t) = sigma(t)/(epsilon) over dot(0) of LDPE melt first gradually increases with time t following the linear viscoelasticity rule in that eta(E)(t) is 3 times the shear viscosity development, 3 eta(t), at low shear rate (gamma) over dot up to a certain critical strain, beyond which eta(E)(t) tended to increase rapidly with t. The behaviour was often referred to as strain-induced hardening. For LDPE melt both sigma(t) and Delta n(t) versus tensile strain epsilon(t) (= (epsilon) over dot(0)t) curves were dependent on (epsilon) over dot(0) in such a manner that the stress optical coefficient C(t) (drop Delta n(t)/sigma(t)) was independent either of (epsilon) over dot(0), epsilon(t) or sigma(t). Addition of UHMWPE up to 10 wt% to LDPE melt increased the levels of both sigma(t) and Delta n(t), but the tendency of strain-induced hardening was reduced. The C(t) was again independent either of (epsilon) over dot(0), epsilon(t) or sigma(t) and also essentially independent of molecular weight (MW) and its distribution (MWD) or the blend ratio. For both LDPE and the blends the C(t) value roughly agreed with that (= 2.2 x 10(-9)Pa(-1)) reported for shear flow experiments, thus confirming the validity of the so far established stress optical rule.