diff --git a/doc/notes/ReleaseNotes b/doc/notes/ReleaseNotes
index 1dd4de16..39d31c49 100644
--- a/doc/notes/ReleaseNotes
+++ b/doc/notes/ReleaseNotes
@@ -2425,3 +2425,7 @@ Put cap on maximum ambient gradient to avoid over-extrapolation of
 poorly computed irradiance cache values.
 
 Added robjutil +T option to triangulate all faces in .OBJ description.
+
+Altered BSDF library to have both front and back diffuse transmittance
+values, which can be different.  This was necessary to avoid issues
+caused by measurement and modeling discrepancies.
diff --git a/src/common/bsdf.c b/src/common/bsdf.c
index af808753..e5572fc9 100644
--- a/src/common/bsdf.c
+++ b/src/common/bsdf.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf.c,v 2.58 2020/05/14 19:20:13 greg Exp $";
+static const char RCSid[] = "$Id: bsdf.c,v 2.59 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  bsdf.c
@@ -381,8 +381,10 @@ SDfreeBSDF(SDData *sd)
 	sd->rLambFront.spec.flags = 0;
 	sd->rLambBack.cieY = .0;
 	sd->rLambBack.spec.flags = 0;
-	sd->tLamb.cieY = .0;
-	sd->tLamb.spec.flags = 0;
+	sd->tLambFront.cieY = .0;
+	sd->tLambFront.spec.flags = 0;
+	sd->tLambBack.cieY = .0;
+	sd->tLambBack.spec.flags = 0;
 }
 
 /* Find writeable BSDF by name, or allocate new cache entry if absent */
@@ -649,10 +651,10 @@ SDevalBSDF(SDValue *sv, const FVECT outVec, const FVECT inVec, const SDData *sd)
 		*sv = sd->rLambBack;
 		sdf = sd->rb;
 	} else if (inFront) {
-		*sv = sd->tLamb;
+		*sv = sd->tLambFront;
 		sdf = (sd->tf != NULL) ? sd->tf : sd->tb;
 	} else /* outFront & !inFront */ {
-		*sv = sd->tLamb;
+		*sv = sd->tLambBack;
 		sdf = (sd->tb != NULL) ? sd->tb : sd->tf;
 	}
 	sv->cieY *= 1./M_PI;
@@ -695,7 +697,8 @@ SDdirectHemi(const FVECT inVec, int sflags, const SDData *sd)
 	if ((sflags & SDsampDf+SDsampR) != SDsampDf+SDsampR)
 		hsum = .0;
 	if ((sflags & SDsampDf+SDsampT) == SDsampDf+SDsampT)
-		hsum += sd->tLamb.cieY;
+		hsum += (inVec[2] > 0) ?
+				sd->tLambFront.cieY : sd->tLambBack.cieY;
 					/* gather non-diffuse components */
 	i = (((sflags & SDsampSp+SDsampR) == SDsampSp+SDsampR) &
 			(rdf != NULL)) ? rdf->ncomp : 0;
@@ -748,7 +751,7 @@ SDsampBSDF(SDValue *sv, FVECT ioVec, double randX, int sflags, const SDData *sd)
 		sv->cieY = .0;
 	rdiff = sv->cieY;
 	if ((sflags & SDsampDf+SDsampT) == SDsampDf+SDsampT)
-		sv->cieY += sd->tLamb.cieY;
+		sv->cieY += inFront ? sd->tLambFront.cieY : sd->tLambBack.cieY;
 					/* gather non-diffuse components */
 	i = nr = (((sflags & SDsampSp+SDsampR) == SDsampSp+SDsampR) &
 			(rdf != NULL)) ? rdf->ncomp : 0;
@@ -784,12 +787,13 @@ SDsampBSDF(SDValue *sv, FVECT ioVec, double randX, int sflags, const SDData *sd)
 	randX -= rdiff;
 					/* diffuse transmission? */
 	if ((sflags & SDsampDf+SDsampT) == SDsampDf+SDsampT) {
-		if (randX < sd->tLamb.cieY) {
-			sv->spec = sd->tLamb.spec;
-			SDdiffuseSamp(ioVec, !inFront, randX/sd->tLamb.cieY);
+		const SDValue	*sdt = inFront ? &sd->tLambFront : &sd->tLambBack;
+		if (randX < sdt->cieY) {
+			sv->spec = sdt->spec;
+			SDdiffuseSamp(ioVec, !inFront, randX/sdt->cieY);
 			goto done;
 		}
-		randX -= sd->tLamb.cieY;
+		randX -= sdt->cieY;
 	}
 					/* else one of cumulative dist. */
 	for (i = 0; i < n && randX >= cdarr[i]->cTotal; i++)
diff --git a/src/common/bsdf.h b/src/common/bsdf.h
index 23f6fccb..5ccac34c 100644
--- a/src/common/bsdf.h
+++ b/src/common/bsdf.h
@@ -1,4 +1,4 @@
-/* RCSid $Id: bsdf.h,v 2.27 2020/05/14 19:20:13 greg Exp $ */
+/* RCSid $Id: bsdf.h,v 2.28 2021/03/27 17:50:18 greg Exp $ */
 /*
  *  bsdf.h
  *  
@@ -126,7 +126,8 @@ typedef struct {
 	double		dim[3];		/* width, height, thickness (meters) */
 	SDValue		rLambFront;	/* diffuse front reflectance */
 	SDValue		rLambBack;	/* diffuse rear reflectance */
-	SDValue		tLamb;		/* diffuse transmission */
+	SDValue		tLambFront;	/* diffuse front transmittance */
+	SDValue		tLambBack;	/* diffuse back transmittance */
 	SDSpectralDF	*rf, *rb;	/* non-diffuse BRDF components */
 	SDSpectralDF	*tf, *tb;	/* non-diffuse BTDF components */
 } SDData;
diff --git a/src/common/bsdf_m.c b/src/common/bsdf_m.c
index 38e3c907..28f1a95b 100644
--- a/src/common/bsdf_m.c
+++ b/src/common/bsdf_m.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf_m.c,v 3.40 2021/03/27 16:40:46 greg Exp $";
+static const char RCSid[] = "$Id: bsdf_m.c,v 3.41 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  bsdf_m.c
@@ -709,10 +709,8 @@ SDloadMtx(SDData *sd, ezxml_t wtl)
 					/* separate diffuse components */
 	sd->rf = extract_diffuse(&sd->rLambFront, sd->rf);
 	sd->rb = extract_diffuse(&sd->rLambBack, sd->rb);
-	if (sd->tb != NULL)
-		sd->tb = extract_diffuse(&sd->tLamb, sd->tb);
-	if (sd->tf != NULL)
-		sd->tf = extract_diffuse(&sd->tLamb, sd->tf);
+	sd->tf = extract_diffuse(&sd->tLambFront, sd->tf);
+	sd->tb = extract_diffuse(&sd->tLambBack, sd->tb);
 					/* return success */
 	return SDEnone;
 }
diff --git a/src/common/bsdf_t.c b/src/common/bsdf_t.c
index 4deb60b5..5a8cbb6e 100644
--- a/src/common/bsdf_t.c
+++ b/src/common/bsdf_t.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf_t.c,v 3.49 2021/03/27 15:53:01 greg Exp $";
+static const char RCSid[] = "$Id: bsdf_t.c,v 3.50 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  bsdf_t.c
@@ -1430,10 +1430,8 @@ SDloadTre(SDData *sd, ezxml_t wtl)
 					/* separate diffuse components */
 	extract_diffuse(&sd->rLambFront, sd->rf);
 	extract_diffuse(&sd->rLambBack, sd->rb);
-	if (sd->tb != NULL)
-		extract_diffuse(&sd->tLamb, sd->tb);
-	if (sd->tf != NULL)
-		extract_diffuse(&sd->tLamb, sd->tf);
+	extract_diffuse(&sd->tLambFront, sd->tf);
+	extract_diffuse(&sd->tLambBack, sd->tb);
 					/* return success */
 	return SDEnone;
 }
diff --git a/src/common/loadbsdf.c b/src/common/loadbsdf.c
index c3c6a58c..1a1c3e12 100644
--- a/src/common/loadbsdf.c
+++ b/src/common/loadbsdf.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: loadbsdf.c,v 3.12 2020/05/14 19:20:13 greg Exp $";
+static const char RCSid[] = "$Id: loadbsdf.c,v 3.13 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  * Simple interface for loading BSDF, Radiance-specific search
@@ -58,8 +58,8 @@ loadBSDF(char *fname)
 						/* simple checks */
 	checkDF(sd->name, sd->rLambFront.cieY, sd->rf, "front reflection");
 	checkDF(sd->name, sd->rLambBack.cieY, sd->rb, "rear reflection");
-	checkDF(sd->name, sd->tLamb.cieY, sd->tf, "front transmission");
-	checkDF(sd->name, sd->tLamb.cieY, sd->tb, "back transmission");
+	checkDF(sd->name, sd->tLambFront.cieY, sd->tf, "front transmission");
+	checkDF(sd->name, sd->tLambBack.cieY, sd->tb, "back transmission");
 #ifdef DEBUG
 {
 float	rgb[3];
@@ -68,8 +68,10 @@ ccy2rgb(&sd->rLambFront.spec, sd->rLambFront.cieY, rgb);
 fprintf(stderr, "Front diffuse RGB: %.4f %.4f %.4f\n", rgb[0], rgb[1], rgb[2]);
 ccy2rgb(&sd->rLambBack.spec, sd->rLambBack.cieY, rgb);
 fprintf(stderr, "Back diffuse RGB: %.4f %.4f %.4f\n", rgb[0], rgb[1], rgb[2]);
-ccy2rgb(&sd->tLamb.spec, sd->tLamb.cieY, rgb);
-fprintf(stderr, "Diffuse RGB transmittance: %.4f %.4f %.4f\n", rgb[0], rgb[1], rgb[2]);
+ccy2rgb(&sd->tLambFront.spec, sd->tLamb.cieY, rgb);
+fprintf(stderr, "Front diffuse RGB transmittance: %.4f %.4f %.4f\n", rgb[0], rgb[1], rgb[2]);
+ccy2rgb(&sd->tLambBack.spec, sd->tLamb.cieY, rgb);
+fprintf(stderr, "Back diffuse RGB transmittance: %.4f %.4f %.4f\n", rgb[0], rgb[1], rgb[2]);
 if (sd->rf)
 fprintf(stderr, "Maximum direct hemispherical front reflection: %.3f%%\n",
 sd->rf->maxHemi*100.);
diff --git a/src/common/testBSDF.c b/src/common/testBSDF.c
index 5dfe197a..b854eba2 100644
--- a/src/common/testBSDF.c
+++ b/src/common/testBSDF.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: testBSDF.c,v 1.15 2019/12/28 18:05:14 greg Exp $";
+static const char RCSid[] = "$Id: testBSDF.c,v 1.16 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  * Simple test program to demonstrate BSDF operation.
@@ -126,13 +126,14 @@ main(int argc, char *argv[])
 						bsdf->rb->maxHemi);
 			if (bsdf->tf)
 				printf("Peak front hemispherical transmittance: %.3e\n",
-						bsdf->tLamb.cieY + bsdf->tf->maxHemi);
+						bsdf->tLambFront.cieY + bsdf->tf->maxHemi);
 			if (bsdf->tb)
 				printf("Peak back hemispherical transmittance: %.3e\n",
-						bsdf->tLamb.cieY + bsdf->tb->maxHemi);
+						bsdf->tLambBack.cieY + bsdf->tb->maxHemi);
 			printXYZ("Diffuse Front Reflectance: ", &bsdf->rLambFront);
 			printXYZ("Diffuse Back Reflectance: ", &bsdf->rLambBack);
-			printXYZ("Diffuse Transmittance: ", &bsdf->tLamb);
+			printXYZ("Diffuse Front Transmittance: ", &bsdf->tLambFront);
+			printXYZ("Diffuse Back Transmittance: ", &bsdf->tLambBack);
 			break;
 		case 'Q':			/* query BSDF value */
 			if (!bsdf)
diff --git a/src/cv/bsdf2klems.c b/src/cv/bsdf2klems.c
index da331f28..4ee28690 100644
--- a/src/cv/bsdf2klems.c
+++ b/src/cv/bsdf2klems.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf2klems.c,v 2.32 2020/11/13 19:21:11 greg Exp $";
+static const char RCSid[] = "$Id: bsdf2klems.c,v 2.33 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  * Load measured BSDF interpolant and write out as XML file with Klems matrix.
@@ -277,7 +277,7 @@ eval_bsdf(const char *fname)
 	    }
 	}
 						/* front transmission */
-	if (bsd.tf != NULL || bsd.tLamb.cieY > .002) {
+	if (bsd.tf != NULL || bsd.tLambFront.cieY > .002) {
 	    input_orient = 1; output_orient = -1;
 	    cfp[CIE_Y] = open_component_file(CIE_Y);
 	    if (bsd.tf != NULL && bsd.tf->comp[0].cspec[2].flags) {
diff --git a/src/cv/bsdf2rad.c b/src/cv/bsdf2rad.c
index 8317b679..64e507a2 100644
--- a/src/cv/bsdf2rad.c
+++ b/src/cv/bsdf2rad.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf2rad.c,v 2.37 2021/02/12 00:53:56 greg Exp $";
+static const char RCSid[] = "$Id: bsdf2rad.c,v 2.38 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  Plot 3-D BSDF output based on scattering interpolant or XML representation
@@ -752,9 +752,10 @@ main(int argc, char *argv[])
 				overall_min = myBSDF.rLambFront.cieY/PI;
 			if (back_comp & SDsampR && myBSDF.rLambBack.cieY < overall_min*PI)
 				overall_min = myBSDF.rLambBack.cieY/PI;
-			if ((front_comp|back_comp) & SDsampT &&
-					myBSDF.tLamb.cieY < overall_min*PI)
-				overall_min = myBSDF.tLamb.cieY/PI;
+			if (front_comp & SDsampT && myBSDF.tLambFront.cieY < overall_min*PI)
+				overall_min = myBSDF.tLambFront.cieY/PI;
+			if (back_comp & SDsampT && myBSDF.tLambBack.cieY < overall_min*PI)
+				overall_min = myBSDF.tLambBack.cieY/PI;
 		}
 		set_minlog();
 		if (!build_wBSDF(&myBSDF))
diff --git a/src/cv/bsdf2rado.c b/src/cv/bsdf2rado.c
index c30475f1..8d0d1032 100644
--- a/src/cv/bsdf2rado.c
+++ b/src/cv/bsdf2rado.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: bsdf2rado.c,v 2.1 2018/06/07 15:39:36 greg Exp $";
+static const char RCSid[] = "$Id: bsdf2rado.c,v 2.2 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  Plot 3-D BSDF output based on scattering interpolant or XML representation
@@ -75,9 +75,10 @@ main(int argc, char *argv[])
 			bsdf_min = myBSDF.rLambFront.cieY/M_PI;
 		if (myBSDF.rb != NULL && myBSDF.rLambBack.cieY < bsdf_min*M_PI)
 			bsdf_min = myBSDF.rLambBack.cieY/M_PI;
-		if ((myBSDF.tf != NULL) | (myBSDF.tb != NULL) &&
-				myBSDF.tLamb.cieY < bsdf_min*M_PI)
-			bsdf_min = myBSDF.tLamb.cieY/M_PI;
+		if (myBSDF.tf != NULL && myBSDF.tLambFront.cieY < bsdf_min*M_PI)
+			bsdf_min = myBSDF.tLambFront.cieY/M_PI;
+		if (myBSDF.tb != NULL && myBSDF.tLambBack.cieY < bsdf_min*M_PI)
+			bsdf_min = myBSDF.tLambBack.cieY/M_PI;
 		if (doTrans && (myBSDF.tf == NULL) & (myBSDF.tb == NULL)) {
 			fprintf(stderr, "%s: no transmitted component in '%s'\n",
 					progname, argv[1]);
diff --git a/src/rt/m_bsdf.c b/src/rt/m_bsdf.c
index ed0fba01..b86370e2 100644
--- a/src/rt/m_bsdf.c
+++ b/src/rt/m_bsdf.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: m_bsdf.c,v 2.61 2020/07/09 17:32:31 greg Exp $";
+static const char RCSid[] = "$Id: m_bsdf.c,v 2.62 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  *  Shading for materials with BSDFs taken from XML data files
@@ -195,7 +195,8 @@ compute_through(BSDFDAT *ndp)
 	}
 	if (vypeak*tomsurr < peak_over*bright(vsurr)*ns)
 		return;				/* peak not peaky enough */
-	if ((vypeak/ns - ndp->sd->tLamb.cieY*(1./PI))*tomsum <= .001)
+	if ((vypeak/ns - (ndp->vray[2] > 0 ? ndp->sd->tLambFront.cieY
+			: ndp->sd->tLambBack.cieY)*(1./PI))*tomsum <= .001)
 		return;				/* < 0.1% transmission */
 	copycolor(ndp->cthru, vpeak);		/* already scaled by omega */
 	multcolor(ndp->cthru, ndp->pr->pcol);	/* modify by pattern */
@@ -255,10 +256,15 @@ direct_specular_OK(COLOR cval, FVECT ldir, double omega, BSDFDAT *ndp)
 			return(0);	/* all diffuse */
 		sv = ndp->sd->rLambBack;
 		break;
-	default:
+	case 1:
 		if ((ndp->sd->tf == NULL) & (ndp->sd->tb == NULL))
 			return(0);	/* all diffuse */
-		sv = ndp->sd->tLamb;
+		sv = ndp->sd->tLambFront;
+		break;
+	case 2:
+		if ((ndp->sd->tf == NULL) & (ndp->sd->tb == NULL))
+			return(0);	/* all diffuse */
+		sv = ndp->sd->tLambBack;
 		break;
 	}
 	if (sv.cieY > FTINY) {
@@ -696,7 +702,7 @@ m_bsdf(OBJREC *m, RAY *r)
 		}
 	}
 						/* diffuse transmittance */
-	cvt_sdcolor(nd.tdiff, &nd.sd->tLamb);
+	cvt_sdcolor(nd.tdiff, hitfront ? &nd.sd->tLambFront : &nd.sd->tLambBack);
 	if (m->oargs.nfargs >= 9) {
 		setcolor(ctmp, m->oargs.farg[6],
 				m->oargs.farg[7],
diff --git a/src/util/cmbsdf.c b/src/util/cmbsdf.c
index 82658bc5..20ec5849 100644
--- a/src/util/cmbsdf.c
+++ b/src/util/cmbsdf.c
@@ -1,5 +1,5 @@
 #ifndef lint
-static const char RCSid[] = "$Id: cmbsdf.c,v 2.9 2021/01/19 23:32:00 greg Exp $";
+static const char RCSid[] = "$Id: cmbsdf.c,v 2.10 2021/03/27 17:50:18 greg Exp $";
 #endif
 /*
  * Load and convert BSDF into color coefficient matrix representation.
@@ -157,8 +157,11 @@ cm_loadBTDF(const char *fname)
 	ec = SDloadFile(&myBSDF, fname);
 	if (ec)
 		error(USER, transSDError(ec));
-	ccy2rgb(&myBSDF.tLamb.spec, myBSDF.tLamb.cieY/PI, diffBTDF);
 	recip = (myBSDF.tb == NULL);
+	if (recip)
+		ccy2rgb(&myBSDF.tLambFront.spec, myBSDF.tLambFront.cieY/PI, diffBTDF);
+	else
+		ccy2rgb(&myBSDF.tLambBack.spec, myBSDF.tLambBack.cieY/PI, diffBTDF);
 	tdf = recip ? myBSDF.tf : myBSDF.tb;
 	if (tdf == NULL) {		/* no non-Lambertian transmission? */
 		SDfreeBSDF(&myBSDF);