diff --git a/shared/src/shapes/cylinder.rs b/shared/src/shapes/cylinder.rs
index b064123..5489a85 100644
--- a/shared/src/shapes/cylinder.rs
+++ b/shared/src/shapes/cylinder.rs
@@ -14,7 +14,7 @@ use crate::{gamma, Float, PI};
 
 use crate::core::geometry::{SqrtExt, Tuple};
 use crate::utils::interval::Interval;
-use crate::utils::math::{clamp, difference_of_products, lerp, square};
+use crate::utils::math::{clamp, difference_of_products, lerp, radians, square};
 use core::mem;
 use num_traits::Float as NumFloat;
 
@@ -43,9 +43,11 @@ impl CylinderShape {
     ) -> Self {
         Self {
             radius,
-            z_min,
-            z_max,
-            phi_max,
+            // pbrt: `zMin(std::min(zMin, zMax)), zMax(std::max(zMin, zMax)),
+            // phiMax(Radians(Clamp(phiMax, 0, 360)))`. phiMax arrives in DEGREES.
+            z_min: z_min.min(z_max),
+            z_max: z_min.max(z_max),
+            phi_max: radians(clamp(phi_max, 0., 360.)),
             render_from_object,
             object_from_render,
             reverse_orientation,
diff --git a/shared/src/shapes/disk.rs b/shared/src/shapes/disk.rs
index e681842..034f803 100644
--- a/shared/src/shapes/disk.rs
+++ b/shared/src/shapes/disk.rs
@@ -7,7 +7,7 @@ use crate::core::shape::{
     QuadricIntersection, ShapeIntersection, ShapeSample, ShapeSampleContext, ShapeTrait,
 };
 use crate::utils::interval::Interval;
-use crate::utils::math::square;
+use crate::utils::math::{clamp, radians, square};
 use crate::utils::sampling::sample_uniform_disk_concentric;
 use crate::utils::Transform;
 use crate::{Float, PI};
@@ -40,7 +40,11 @@ impl DiskShape {
             radius,
             inner_radius,
             height,
-            phi_max,
+            // pbrt: `phiMax(Radians(Clamp(phiMax, 0, 360)))`. The parameter arrives in
+            // DEGREES (default 360); storing it raw made `area()` 360/2pi = 57.3x too
+            // large, so the sampling pdf was 57.3x too small and every direct-lighting
+            // contribution from a disk area light was 57.3x too bright.
+            phi_max: radians(clamp(phi_max, 0., 360.)),
             render_from_object: render_from_object.clone(),
             object_from_render,
             reverse_orientation,